H&K NFA Conversions

MM23 image by H. Hardin

The following article is republished with permission of Small Arms Review and the author, Thomas T. Hoel of Tactical Advantage.  The articles were originally published as a 3-part series in the April, May and June, 2000 issues of SAR.  

The Mechanics of the Heckler & Koch NFA Conversions

By Thomas T. Hoel
Photos by James Bardwell, Tom Hoel and Dan Shea

With the Civilian NFA Weapons collecting market in a continual stage of flux, one thing remains constant. Those firearms of the general type almost universally known as “H&K” models remain THE most favored and desirable amongst those collectors of firearms of the “modern”, post World War II era.

Their almost mythic popularity and desirability are not without pitfalls however, as unless one is well versed in what is available in terms of the mechanics and cosmetics of the available versions, it can come as a rude awakening to discover what one has acquired is not necessarily what one expected! The fact that these guns, as a class, are some of the priciest civilian legal semi-automatics makes it incumbent upon the owner, or individual considering ownership, to grasp the many significant factors that determine relative value of the vastly different incarnations of these guns. With very few exceptions, those “H&K” type machine guns available for civilian purchase as fully transferable Title II NFA weapons all began life as Title I semi-automatics. This the basis for the most confusion, as there were many different routes taken to convert the guns into functioning selective fire full-automatics. These converted semi-auto guns are often surrounded by confusion and misunderstanding, and it is that problem that this article will address in detail. 


Factory HK33E machine gun showing push-pin, swing down type
 of trigger group attachment.


Even the common identifying term of “H&K” is a misnomer since an examination of the history and pedigree of this general firearm design will reveal that the salient features of its design were well established during the late WWII years in the work of the famous Mauser-Werke firm on the evolutionary prototype Sturmgewehr-45.  Only after a period of considerable movement around continental Europe post-war did this basic design eventually find a permanent home with the reconstituted pre-War Mauser factory in the guise of the then new West German firm of Heckler & Koch GmbH, and its ultimate final conversion to the then new NATO 7.62mm caliber. This occurred, after having been initially designed and experimentally first produced in post-war Spain by the Government entity CETME during the period 1949-1956, with the engineering guidance of Ex-Mauser-Werke employees who had fled Post-War Germany and settled in Spain. After a hurried initial adoption of the new rifle in 1957 by both the Spanish and West German militaries, the Spanish and West German Governments co-operated for a time on the final development and perfection of the basic operating system until it became clear that there existed differing requirements for a new service rifle in each country. Development had begun in Spain not only because the prominent ex-Mauser-Werke engineers were presently residing there, but also because West Germany had initially adopted the also then new FN FAL rifle as the Gewehr-1, the “G1" FAL. Only when it became quite clear that FN-Herstal was not going to allow a production licensing agreement to take place, allowing for West German production as opposed to direct contract purchase from FN, (...owing to a lingering bitterness over NAZI Germany’s roughshod run over Belgium) did the West German Government begin in earnest a search for a suitable equivalent rifle design that could be domestically produced. The differing specifications extant in Spain and West Germany by 1963 caused termination of the second joint-development agreement and ultimately resulted in the production of two similar, but different versions of the perfected original CETME 7.62mm NATO caliber Modelo “C” rifle design. The CETME produced versions issued to the Spanish military were in a slightly different form than the now ubiquitous German Army standard Gewehr 3, or “G3" as it became universally known. Parts and fittings were not fully interchangeable, though fundamentally similar.

That Heckler & Koch has become synonymous with the particular mechanical design type is due more to that firm’s aggressive marketing and promotional strategies, than any original conception of the design parameters. Although since the pre-War Mauser-Werke firm resurfaced as H&K GmbH, there were no doubt a multitude of significant individuals still on hand from the time of its wartime conception, and the return of the original engineering team to West Germany from Spain/CETME no doubt cemented HK GmbH as the preeminent production facility of this design weapon.  It goes without saying too that H&K GmbH has considerably mined the potential for expansion within the basic design parameters in the ensuing years, producing a vast family of mechanically similar weapons. Clearly this potential is owed in large part to the soundness of the original concept which allowed an inherent capacity for expansion into differing calibers and applications.

So for purposes of discussion, when we talk about an “H&K” type firearm we are using that term in the general sense, an even more important distinction when it is known that there have been more than a dozen countries which have produced guns to the “H&K” military pattern(s) since the mid-Fifties, with a few even producing guns in the semi-automatic configuration for intended sale to law enforcement or civilian markets worldwide where such sales were lawful.  “H&K” type guns made by H&K GmbH licensees to the pattern of the semi-automatic versions made and marketed by the West German firm in the USA have also been imported here in substantial numbers over the years. Imports of these semi-auto versions were clones of the HK91 .308 caliber rifle and came from Greece (Springfield Armory SAR-3/SAR 8) and Portugal (FMP XG3S; various importers). H&K GmbH had the sole honor of manufacturing and offering for import through their US based sales and marketing arm, HK USA, Inc., civilian legal sporting guns in the .233 Remington and 9mm Para calibers with their models HK43, HK93, HK94, and their  “pistol” version gun, the SP89.

Foreign Licensee production and importation of semi-automatic sporting arms was limited to the .308 caliber class weapons, the “HK91" clones. No COM-BLOC 7.62x39mm M43 caliber sporting weapons were ever imported into the USA by anyone, though H&K did catalog a few versions of their guns in that caliber for military sales. (NOTE: as this article was being prepared it appeared as though a new US based concern, Special Weapons LLC, has obtained ATF approval to begin manufacture and sale of a “US made” version of the HK91 & HK94 type semi-auto Sporting rifles using receivers manufactured in the USA, with other major components obtained from foreign Government Licensee producers of HK type weapons, together with enough US manufactured minor components to qualify the guns as being “Made in the USA” according to the new Import restrictions currently in effect. The information available seems to indicate that these new made receivers will be close copies of the original design, though not identical in all respects)

“I want an HK machine gun!”

“I want an H&K machine gun, but it has to be fully transferable...What are my choices?”  Exactly two. In order for any H&K type machine gun to be classed as fully transferable to individuals under the auspices of the National Firearms Act, it must have either been imported prior to the enactment of the GCA ’68, or else “manufactured” here by either a licensed Class II Manufacturer or an individual filing an ATF Form 1 making application, prior to the passage of the Volkmer-McClure Firearms Owners Protection Act of 1986.  


Two basic receiver attachment types.

On top, a factory original machine gun with push-pin attachment, and on bottom a
 factory original semi-auto receiver with the clip on attaching point.  The semi-auto
 has been modified to fit a flapper style magazine release to emulate the factory MP5.


What this means in reality terms is that almost all of the fully transferable H&K type machine guns available for purchase by individuals are those guns converted from semi-automatic versions originally imported as Title I guns, since only a very, very few of the factory original machine guns were ever imported prior to GCA ‘68. And the vast majority of those Pre-68 factory guns being imported were imported either in an attempt at attracting Law Enforcement and/or Government sales, or for sales directly to those entities, which means they may never have moved into the arena of normal commerce. There were also an extremely limited number of H&K weapons imported into this country Pre-1968 by domestic Firearms companies who sponsored the designs under their own name and model designations in US trials, and would have built and supported them had the contracts ever been awarded; the guns from the firm of Harrington & Richardson are most well known, and these guns eventually made it into many private collections when the H&R Museum collection was sold off. It is known, also, that a certain small number of private individuals did in fact personally import these factory original guns when it was still legal to do so, so they represent a legitimate source of fully transferable factory guns, though their numbers are unfortunately extremely small and their collectors’ value is understandably extremely high. So if we wish to discount these select few fully transferable factory original machine guns on grounds of pricing or rarity we are left with the vastly more common NFA registered conversions of their semi-automatic cousins, which should indicate our choices are narrowed considerably, but unfortunately that is not the case!

What causes the confusion about these “conversion guns” is that they can exist in a myriad of differing mechanical and cosmetic configurations, and that within a specific sub-type of conversion method there are even differences. All conversions are NOT the same, or equal! Within this framework we will be examining the differing mechanical methods used to perform the conversion of a semi-automatic weapon into a select-fire near copy of the factory machine gun. Previous works have examined the vast catalogs of offerings from those who actually performed the conversions on these guns when it was a legal enterprise; this treatment will be limited to an examination of the mechanical means by which these conversions reached a means to an end of obtaining an H&K type machine gun available for individual transfer and ownership. Since this is a specific treatment of the conversions of those semi-automatic guns originally imported as Title I Sporters, by nature we will only be considering those guns known commonly as HK91 (HK41), HK93 (HK43), HK94, and SP89 types.  


Two SEF style plastic trigger pack housings.

On the right is an unaltered factory original pushpin mounting unit, and on the left
 is a modified push-pin housing altered to fit a semi-auto clip on receiver mounting.
  (Note the relief cuts to the face of the front metal, and the fake pushpin installed on
 the modified unit.)


There were other special versions of the basic rifles imported and sold on a very limited basis by HK USA Versions of the .308 NATO caliber HK91 known as the HK911, SR-9, SR-9T & SR-9TC were imported, as were a dramatically small number of a special “Marksman's” version of the HK94 9mm rifle. Due to import restriction factors of when they were imported, these guns were originally priced considerably higher than their standard counterparts. They were also configured in vastly different styles than the standard rifles, so it is extremely doubtful if any of these guns were ever used as the host gun for a select-fire conversion, although with the increasing rarity and escalating prices of “pre-Ban” guns it is quite possible they may one day be used as the host weapon for a conversion project using a NFA registered sear or trigger pack.  (Due to extreme cost and mechanical differences we will not be examining the PSG-1 Precision Marksman’s’ Rifle, although it too was imported as a semi-automatic Civilian legal Rifle.)

As far as the non-West German origin imported Sporter rifles go, there may have been different designations or appellations given to the various clones of the “HK91" type rifle by non-H&K GmbH producers, but the guns are mechanically the same and will be considered on a parallel basis to the West German produced HK91.

The Receiver: Where It all Begins.

 Machine gun or Semi-automatic, ALL H&K type receivers are the same! Lets take a long look at THAT statement because the receiver of these conversion guns is often the point of the most contention when it comes to understanding, and defining, a particular conversion method, or its official legal status under the Regulations of the National Firearms Act. And sometimes, more importantly, interpretations of those Regulations by BATF Technology Branch.

To understand the above statement in the context of a converted semi-auto Sporter gun into a mechanically viable select-fire version, one needs to understand how the receiver component is actually manufactured. All H&K type receivers are made by high pressure sheet metal press-forming under a mandrel... commonly called “stamping”. This finished formed ‘blank’, formed in the shape of both sides of the receiver laid out flat, is then carefully folded and seam-welded into the common finished form or ‘receiver body’. Part of the forming process is the punching or indexing of the various holes in the receiver required for assembly, or alignment of the sub-components or assemblies of the completed weapon. Up through this point ALL H&K type receivers, within a common caliber grouping, are the same during the manufacturing process.

One of these sub-assemblies is the entire fire-control group, which contains the “fire-control trigger pack” which is mounted inside the trigger grip housing, and that together are commonly identified as the “trigger group” on H&K type guns. (For purposes of this discussion, we need to standardize some terminology regarding the fire control components, as even the factory official nomenclature can be confusing at times. The fire control group as mentioned above is composed of two distinct sub-assemblies: the outer grip frame housing, which can be either of full sheet metal construction or in later versions of a hard plastic molded over an inner metal frame, and the internal sheet metal trigger pack which contains and provides a secure mounting for the individual mechanical component pieces which provide the fire control functions.) These fire control groups can come in many different styles as will be examined, but in basic terms they were produced in either of three major categories. Originally with a “slab-sided” full sheet metal exterior housing with a separate contoured palm grip piece offering the standard “S-E-F” (Safe, Semi-auto, Full-auto) fire control selections, and later a cast metallic housing with a molded in ergonomic finger/palm grip. With the introduction of the full plastic style ergonomic featured housings, a wide range of fire control selections was offered including groups with standard “S-E-F”, 2 or 3 shot burst features either alone or with semi-automatic selection. The latest groups are even offering a second external configuration of full plastic housing resorting back to a ‘smooth’ grip contour while introducing an ambidextrous control selector feature to the full range of fire modes.

On most factory original machine-gun versions the fire-control group is mounted by, and pivots on in the front, a horizontally sliding assembly pin front and rear. (Some notable factory original exceptions to this trigger group mounting rule are machine guns such as the HK21E and G41, which for specific and intended design purposes, are equipped with a non-pivot pin type front trigger group attaching mount. A contrary exception to the general rule is the MSG90 Selected Marksman’s’ Rifle; though the MSG90 is intended as a semi-automatic weapon it is designed with a front pivot-pin mounting attachment for the semi-automatic fire control group, and as such, BATF has classified this particular weapon as being legally classed as a “Machine Gun” even though it is incapable of automatic fire in its intended form.) This front assembly/pivot pin is aligned and held by a set of indexed holes located on a rounded protrusion of the receiver bottom at the rear of the magazine housing upper area. It is mated with a permanently affixed (usually welded, though press fitting is sometimes used) fitted bushing on the inside of the two pressed steel sides of the receiver stamping for the assembly/pivot pin to pass through and bear against. By this method the trigger housing can be held fast in the proper position on the receiver, and also pivot downward for disassembly or cleaning. This arrangement is commonly known as the “push-pin trigger group assembly”, or “swing-down trigger housing” type. It is the original and most common method of attachment of the trigger housing on factory machine-guns. This feature alone, the holes and mated bushing for the front assembly/pivot pin, has been determined to be the sole defining feature by BATF Technology Branch as to what constitutes in a legally defining sense a “Machine Gun” receiver for a H&K type weapon under the National Firearms Act.

A “semi-automatic” receiver intended for a Title I semi-automatic Sporter rifle is taken from the same assembly line as its machine gun cousins. It is only after the receiver is completely formed, does any distinction arise. The ONLY difference in the receiver of a semi-automatic gun is in its method of attaching the trigger housing, or “trigger group”. Within a selected caliber grouping of weapons there are NO internal receiver differences between, say, a select fire G3 and the semi-auto-only HK91!! This is significant.

On semi-automatic-only receivers, in order to pass the scrutiny of US import restrictions intended to prevent easy or rapid conversion to semi-automatic fire mode, H&K had to devise a suitable method of preventing attachment of the standard factory ‘push-pin, swing-down’ type trigger group/trigger housing used on factory original machine guns. It also had to be able to correctly hold alignment for the trigger pack mechanical components on the receiver. Further, it had to be able to be developed from the basic receiver component already being produced for the machine gun production line, as to re-tool for completely different stamping dies would be economically unfeasible for the limited market represented by Civilian (or Law Enforcement) sales in areas where such markets existed.

This redesign was accomplished by altering the forward point of attachment of the trigger housing group, from a bushing lined set of holes through the lower rear of the magazine well combined with a transverse mounted push-through retaining pin, to a transverse oriented ledge at the lower rear of the magazine well fitting into a rectangular cut out in the lower front of the semi-automatic only trigger housing (and a suitably modified fire control pack). This rectangular cut out in effect ‘clips on’ the ledge, or lip of the rear of the magazine well. This ledge, or ‘lip’ is actually a separate U-shaped machined piece which is welded around the location of the normal pivot holes and bushing, and in some versions is attached after the normal pivot pin holes are punched into the receiver stamping, leaving the holes clearly visible from the inside, though the pin bushing is not present, and the holes themselves are blocked with a welded in bar.  Guns produced in Greece by Hellenic Arms S.A. (Springfield SAR3/8) are known to be made this way. Due to this, on semi-auto trigger frame housings the front semi-circular “ears”, which contain the pivot pin holes on machine gun versions, are removed leaving a squared-off shape to the front of the trigger housing which does not fully contour into the lower portions of the receiver, leaving a cosmetic mismatch.

The other significant difference that this causes functionally between the factory machine gun receiver and the semi-auto-only receiver, is that since the front pivot-pin holes and bushing are not available, the standard machine gun style “flapper type” magazine release cannot be installed in the usual manner. This “flapper” is actually an additional mag release point which pivots on the push-pin bushing in the machine gun style receiver, and by contacting a wedge shaped contour on the actual magazine release shaft, causes the shaft to move laterally releasing the mag catch in the same fashion as pushing the catch itself would in the normal manner. As such, all semi-auto-only receivers are produced with only the actual magazine release catch button, though most operators find the “flapper” style release to be more desirable. Conveniently though, many enterprising Class II manufacturers have designed a modification of the factory flapper style lever that passes BATF Technology Branch scrutiny, allowing a functional flapper release to be installed on semi-auto receiver without modifications in the legal sense.

The removal of this U-shaped ledge piece, or its remaining intact, is the MAJOR DEFINING DIFFERENCE IN VALUE of those guns converted into registered machine guns from semi-auto Sporter beginnings. This is simply because with the “clip-on” lower feature removed, the gun reverts back into its full machine gun receiver configuration and can be equipped with a normal swing-down, push-pin type trigger housing (and fire control pack), the same as a factory delivered machine gun came equipped as originally designed. In terms of operating system function, if you can attach a swing down trigger group, there is no remaining mechanical distinction between a semi-auto gun so converted and a true, factory original, machine gun. A semi-auto gun converted in this fashion can use any and all factory original push-pin type replacement trigger groups, and spare parts for such items are plentiful and relatively inexpensive...something that is not necessarily true if the gun was converted by other methods.

All other conversion methods employed were done in an attempt to circumvent this fact that the ‘clip-on’ ledge has been left intact on the individual gun in question. The fact that the receiver of the gun in question may in and of itself be the NFA registered item, as opposed to employing a “machine gun conversion part or device”, is of no consequence in this. It is solely based upon how the gun was mechanically converted ORIGINALLY AT THE TIME OF CONVERSION to a functioning machine gun (and its attendant registration status), that determines its respective monetary value as a transferable NFA weapon.

 Registered Receiver Conversion?  Maybe...maybe not!

This is where a pivotal distinction needs to be made before we are able to go further. As has been alluded to above, NFA converted guns can be most basically categorized two ways. First, there are those guns that have been converted not just to replicate the functionality of a factory selective fire product, but actually physically altered to duplicate the factory machinegun in mechanical function, appearance, and physical dimensions by removing all the defining features of the semi-auto iteration and restoring the features identical to a factory produced machine gun, in effect reverse-engineering the semi-auto design changes. 

Secondly, there are those guns that have been cleverly altered to effectually replicate the selective fire capabilities of a factory produced machine gun, either by employing an NFA Registered conversion part (such as a “Registered HK Auto Sear”), or a legally installed unregistered conversion part or device, while retaining the legally defining features that identify the receiver of the gun as a semi-automatic Title I firearm by definition.

As a general rule these types of conversions retain the single legally defining feature of the clip-on trigger housing attachment, even if the receiver itself was NFA registered as the “machine gun” in the case of those conversions employing an unregistered conversion part. So if the receiver itself was registered as the NFA controlled item, but not mechanically converted at the time of conversion to utilize a swing down lower, the mechanical alterations to achieve semi-automatic fire capability must have been done to other components of the weapon in question.  


Two clip-on style housings.

On the right, a factory machine gun housing is adapted to fit a clip-on
 style mount.  On the left is an unaltered, original semi-auto housing.


Such is the case that the mechanical differences between a semi-auto-only capable rifle and a fully automatic capable rifle can be contained completely within the fire control elements located in the trigger group. It is common to employ the phraseology “conversion part” for a single NFA registered piece such as a “registered HK Auto Sear” when used as the basis of a lawful conversion. And the phraseology of “conversion device” is used in describing an assembly converted as a unitized item utilized as the basis for lawful conversion, such as the adapted factory select fire trigger packs and grip frames. So when we speak of an “unregistered conversion part or device”, or a “NFA registered conversion part or device”, we are relating the fact that within the fire control trigger pack, certain parts not present in the semi-auto-only capable version can be introduced to replicate the functionality of the factory original selective fire version, all without mechanically altering the actual receiver of the gun. This is the case whether we are discussing a “Registered Receiver” conversion where the actual receiver of the gun retains all the defining semi-auto features, yet is NFA registered itself, or if a “Registered Conversion Part” is utilized to convert an existing Title I gun. In both of these legally disparate cases, the receiver of the gun remains unaltered mechanically. All mechanical changes necessary to affect conversion into a functioning selective fire weapon are carried out on the fire control group alone.

Whether registered or unregistered themselves as conversion parts, the introduction of these additional parts into a semi-auto fire control pack is designed to restore the three missing functions present in the selective fire version. First, since these guns fire from the closed bolt and are hammer fired, there must be a mechanical means to prevent hammer fall until the bolt is locked into battery. Secondly, there must be a means to time that hammer fall correctly. And lastly, there must be a means to select the mode of fire desired. In H&K style guns these three requirements are met by the addition of two parts not normally present in the semi-automatic-only fire control pack.

The first and most important of these “missing” parts is what is known in official H&K nomenclature as a “Catch”, but is more commonly understood as an “automatic sear”; this part serves to hold the hammer from falling until mechanically released at the proper time. It is also present to prevent inadvertent or premature hammer fall, in short a secondary safety system. H&K nomenclature specifies that the part performing the disconnector function is known officially as the “sear”, because it functions in both capacities when it is operating independent of the “automatic catch”. The “automatic catch” and “sear” are totally independent mechanical systems, operating in conjunction only with the selector, or “change lever” in factory parlance. With the selector lever in the cyclic fire mode, these two independent systems operate in parallel to allow cyclic fire operation. The automatic catch ALWAYS functions, only when the selector lever is set on cyclic mode does it allow the trigger to lower the sear out of position to prevent the disconnect function. This means the only mechanical system acting on the hammer then is the automatic catch, functioning as the primary sear in this mode.  This is why it has come to be commonly known as the “automatic sear”. (To avoid further confusion we will use the term “sear or automatic sear” when specifying the factory term of “catch”, and disconnector in place of the factory term “sear”. This is due to the universal application of these terms currently in the Civilian NFA community, not to instill confusion with factory nomenclature!!)

The only other required additional “missing” part is what is called the “release lever”. This is a vertically mounted swinging arm located on the forward right side of the trigger pack frame. At its mid-section it is in contact with the extension arm of the “catch”(auto sear). Its upper end is located in the path of travel of the right underside of the bolt carrier which activates its forward movement upon final dwell of the bolt carrier after the bolt has moved into battery, causing release of the catch and hammer fall. 


Two semi-auto clip-on mounting style trigger packs.

On the right, an adapted factory semi auto trigger pack frame with registered
 sear installed.  On the left is an altered factory machine gun pack.


In terms of component parts, these additional two parts are all that distinguishes the two versions of the basic fire control pack, although the actual physical specifications of certain parts vary slightly between the two versions. With these simple additions to the standard semi-auto trigger pack, and minor modifications to a few other small areas discussed below, a functional selective fire conversion can be achieved. The fire control selector switch is identical in both semi-auto and selective fire variants of the guns, and is merely prevented from moving into the full-automatic selected position in the semi-auto trigger groups by a mechanical detent formed into the exterior of the trigger housing.

In real life terms, these so called “conversion parts” usually took the form of a proprietary design of the “catch”, or “auto sear”, although there were a number of variations on that theme.  Besides several variations of “auto sears”, such other items will be encountered as combination release lever/auto sears, or combined auto sear and pivot. In those cases, only the “catch” was legally defined to be the actual “conversion part”; the release lever and required modifications to the trigger frame itself to affect proper installation were not considered in a legal sense. In most installations a factory release lever was utilized without modifications.

When moving into the realm of “NFA registered conversion parts or devices”, it was mechanically almost the very same items and choices, the single legally defined conversion part was simply marked and registered as the “machine gun”, it being the only controlled part. When exploring NFA registered conversion items though another category is sometimes encountered, that being the registered conversion trigger pack itself, as a complete entity. This was the actual stamped metal trigger frame of a stock factory produced MG trigger group complete with all unaltered factory MG parts, suitably modified along with its companion trigger housing to fit the clip-on attachment of a semi-auto receiver, marked and registered as a complete unit, and were known to have been NFA registered as a “conversion device”. While rare, these units are highly desirable as will be shown.

(Just to muddy the waters a bit, there are known a very few “Title I” guns hat have had the receiver trigger housing attachment point originally converted to a push-pin, “swing-down” style while employing a Registered “Auto Sear” as the NFA registered item, leaving the now highly modified Title I receiver in a questionable legal status.  These use a modified semi-auto trigger pack the same as most “Registered Sear” conversions, and ATF is surely not happy with these guns for sneaking through, and their legal status is at best murky.

Additionally, some of the very first “HK91" style semi-automatic rifles imported were actually made with a receiver that was identical to a factory MG, that is it used a push-pin type trigger housing attachment to attach a SEMI-AUTO capable only trigger mechanism, which was nothing more than a select fire trigger pack modified to prevent automatic fire. These guns were even identified on the receiver as “G3", and were imported by Santa Fe Div., Golden State Arms Corp. A select few were snapped up by enterprising Class II’s and some enlightened individuals and converted to transferable machine guns by registering the actual receiver and simply pinning on a standard factory MG trigger group. Such a gun would have doubly interesting value to a collector!)

It can be readily seen then that the most desirable method of conversion was just to fully restore the receiver of a semi-automatic gun back into a machine gun receiver able to use factory original machine gun, push-pin attachment type, swing down trigger groups by removing the ‘clip-on’ lower attachment block and preparing the forward attachment point to accept a pivot-pin attachment. This was most usually accomplished by milling off the ‘clip-on’ ledge, and then align drilling the two holes in the lower rear of the receiver, followed by welding in the pivot-pin bushing. With these modifications accomplished a factory swing down trigger group could be attached in the normal manner, and that combined with installing a factory original bolt carrier (or a modification to the semi-auto version) completed the mechanical alterations needed to produce selective fire. If this method was chosen, at the same time a factory machine gun type “flapper” style magazine release could also be installed in the correct manner, mimicking fully the factory machine guns.

So why weren’t all these guns being converted this way?? Two simple reasons. First, with the surplus parts market the way it is today many people are unaware that things weren’t always as they are now. The availability of foreign machine gun parts and/or parts kits in the quantities they are seen now is a fairly recent phenomenon, due to a positive change in the relevant laws. The ready availability of these parts today from factory produced machine guns makes it much easier to contemplate doing a near exact copy of the factory made gun as many of the needed parts could be simply exchanged. This situation however was definitely not the case during the time frame that most of these semi-automatic guns were being converted and registered as Civilian transferable machine guns. The most common reason that so many Class II manufacturers performed conversions using heavily modified semi-auto parts is that they had no choice! The registering of the semi-auto configured receiver, as the ‘machine gun’ was just the most commonly accepted sequence, after which one went to work on the semi-auto trigger group parts to affect the needed mechanical changes to render automatic fire. Then a further realization, due to a late ATF ruling, that certain dedicated parts used to convert a semi-auto gun to automatic fire mode were also now being legally classed as “machine gun conversion parts” in and of themselves prompted a rush to register these parts alone as “machine guns” in large quantities. It became far more economic in terms of time and manufacturing ability to concentrate on producing these so called “conversion parts” then it did to take the time to locate, obtain, and mechanically convert complete guns. Such was the genesis of the common “Registered H&K Auto Sear”. Thus a time line developed whereby before this ATF ruling most NFA conversions had the actual receiver of the gun registered as the “machine gun”, whether or not the receiver had actually been fully restored into a machine gun receiver, and after the ruling most manufacturers resorted to registering the conversion part, or device, or “kit”. It has become synonymous within the industry to recognize the term “Conversion part” as exclusively meaning a conversion auto sear (the “catch” in HK terminology), whether it is in and of itself NFA registered or not. This is due to the BATF determination and legal classification of this part as a “machine gun” as described above. As will be seen, the other required “parts” for a select fire conversion are not discussed separately in a legal definition.

It was a lucky Class II manufacturer who was able to procure enough original and authentic factory made machine gun parts to perform a complete receiver conversion back into the factory select fire specifications. Those authentic parts that were available were always in short supply and priced accordingly. With current import law, price trends and world market forces, many do not remember that the cost of a conversion job to a semi-auto gun was considered prohibitive if it approached a few hundred dollars!! Thus, factory original parts were often viewed as a serious cost factor. Modifying the original semi-auto configured parts was often done as a matter of course, and economic reality. With H&K type guns it was always parts for the .308 caliber G3 rifles that were in greatest supply and so authentic parts, for say an MP5 SMG, were often nearly impossible to locate with any consistency or price control. This is one reason why there are such a disproportionately low number of registered receiver conversions that were done by a complete restoration to the factory select fire push-pin type, swing down trigger group, specifications.

The other major reason for registered receiver conversions being performed with unregistered conversion parts employed on a NFA registered unaltered semi-auto receiver was due to the simple fact that many of the active Class II manufacturers in the heyday of the conversion craze were less than Journeyman level skilled machinists or Gunsmiths. Compared to the recognized handful of true Artisans who today are active in the H&K repair/modification arena, there were some shops that turned out vast numbers of conversion guns with less-than-perfect machine work on them, and as a consequence many of these same shops quickly realized it was a lot easier to leave the expensive receiver alone and perform the needed mechanical alterations to the more easily worked on fire control packs and trigger housings. The required alterations to the semi-auto bolt carriers of guns from this general pedigree for instance, are often times of dubious quality and in need of rework to achieve reliable functioning.

It is fact that many Class II Manufacturers, and Form 1 approved individuals, decided to employ an altered semi-auto trigger pack for ease of conversion, while registering the receiver itself as the NFA controlled item. This of course left a dilemma as it allowed the existence of a semi-automatic configuration receiver which was registered as a legal machine gun, while producing an unregistered conversion part, which by law was allowed to exist as long as it always existed in physical contact with the NFA registered semi-automatic configuration receiver.

 Into the murky depths...

ATF Technology Branch has quite emphatically ruled that a registered receiver gun, that as a registered conversion using a clip-on style trigger group housing, cannot be later converted to a different method of trigger housing attachment. In other words, a registered receiver conversion originally mechanically converted with a clip-on style trigger pack attachment method cannot later undergo a further alteration to a push-pin, swing down lower attachment provision. Plainly stated, ATF has ruled that the removal of the clip-on ledge piece of metal and subsequent drilling of the two pivot pin holes is, in and of itself, the act of manufacturing a NEW MACHINE GUN, regardless of whether or not the actual receiver is already registered as a machine gun!! While this may seem to make no logical sense at first glance, one must understand why ATF has taken this position and that is because they know that a registered receiver converted gun employing a ‘clip-on’ style trigger housing will have had to have been mechanically converted using a semi-auto fire control pack that has been modified to function in the selective fire mode, or in very rare cases a factory original machine gun pack and trigger frame altered to fit the clip-on attachment of the semi-auto receiver. They know that if you remove the clip-on ledge from the receiver, post initial conversion date, that you can then use a normal factory original machine gun-style fire control pack…all of which leaves the very interesting question of what happened to the original converted semi-auto fire control pack which had been modified to fire in the selective fire mode (or a modified machine gun pack and housing) and could, in and of itself, be used to convert another semi-auto gun!! Because the legal definition of what constitutes a ‘machine gun’ also defines conversion parts, or assemblies, to be ‘machine guns’ in and of themselves this leaves an “extra” unregistered machine gun out there. This is why the subsequent removal of the clip-on ledge, post initial conversion date, is creation of a NEWmachine gun because presumably it frees up an unregistered conversion part (or assembly)!  With a clip-on style trigger housing attachment method the actual NFA registered “machine gun”, as it was originally registered, is legally defined to be the combination of the semi-auto receiver and the original unregistered conversion part...not just the receiver alone!


Two converted selective fire packs:

On top, a converted pack utilizing a registered sear, and on the bottom, is a factory
 machine gun pack adapted to fit a clip on mounting.  Clearly visible are the different
 locations of the sear and pivot pin hole.  The differing geometries are what cause a
 difference in the cyclic rate of fire.


The genesis of this ATF philosophy is derived from the enactment of the Hughes machine gun freeze amendment which was slyly inserted into the otherwise good natured McClure-Volkmer Firearms Owners Protection Act, enacted on that now famous date May 19, 1986. The Hughes amendment forever capped the number of fully transferable machine guns available to civilians, and this fact is what drives the ATF position on further modification of  already registered receivers. Even more interesting in this case was the position ATF Technology Branch took on the status of literally thousands of lawfully NFA registered machine gun conversion parts or kits, most of which were hastily registered shortly before the enactment date of the McClure-Volkmer Act. ATF Technology Branch had initially taken the official position that any lawfully registered machine gun conversion part or kit could be installed in the host firearm at a date later than the initial registration date. This meant that a manufacturer or lawful registrant could hold onto that registered conversion part or kit indefinitely until it was desired to affect the actual installation into a host firearm. Problems soon arose however when ATF Field Agents tasked with checking on the compliance of Class II manufacturers soon discovered that certain of these manufacturers, in a greedy rush to register as many fully transferable conversion items as possible prior to the cut-off date, had been less than forthright in their descriptions of the conversion parts or kits.

Many of these parts or kits fully failed ATF’s basic test for what constitutes a “machine gun conversion part” able to be lawfully NFA registered, in and of itself, as a machine gun. ATF Technology Branch had had to make the decision to allow the classification of certain items as “machine gun conversion parts or kits” based upon the technical possibility that installation of one of these parts of kits, without modification to the actual firearm receiver itself, could affect the alteration of the firearm into a fully automatic mode of fire. This was historically presented once before by the Technology Branch ruling in early 1969 on the NFA status of the so-called “M2 Carbine Conversion kit” after the enactment of the Gun Control Act.  The key element in the allow ability of a “conversion part or kit” was that it had to be able to be installed without any additional modification to the receiver of the host firearm. Any amount of modification was allowed to any other component parts, but not to the actual Title I receiver or component parts defined as the Title I receiver. Unfortunately, many unscrupulous Class II’s, and some Form 1 approved individuals, had simply registered parts or “kits” that could in no way be physically installed without further modifications to the host firearms’ receiver. The now classic “registered AK auto sears” are the best example, although all manner of things were attempted including dead-stock M16 auto sears for “drop-in installation” into AR15 type receivers!! These “conversion parts or kits” were in no way physically possible of being installed without significant alterations to the host firearm receiver, and ATF soon had a major dilemma on its hands since literally thousands of these NFA registered conversion parts or kits now existed and they had to go about defining which ones were lawfully registered, and which ones were erroneously registered. There was also the significant question as to the status of those host guns which had previously had such parts or kits installed. Between May 1986 and December 1987 ATF had grudgingly allowed the installation of these NFA registered conversion parts or kits requiring additional receiver modifications, insisting that any host weapon so converted into a “machine gun” have the registered conversion part or kit “married” to the host receiver by serial number on the relevant registration form effectively forever preventing the future separation of host receiver (now modified) and the registered conversion part(s). This was to prevent subsequent removal of the registered conversion part, freeing up a modified Title I receiver which could accept new parts and continue to function as a machine gun. This concept of future separation is key to understanding the H&K conversion dilemma!!  


Two clip-on style housings.

On top is a semi-auto pack to fit a clip-on style mount.  On the bottom
 is an altered FBI S-F pack frame which was originally configured the
 same as a machine gun pack.  The FBI S-F ambidextrous packs are
 very uncommon and command a premium price.


At the Las Vegas SHOT Show in January 1988, ATF declared in their Industry Seminar that those Title I semi-auto guns converted using a registered machine gun conversion kit, but which had required receiver modifications to be contraband and illegal! They further stated that those guns already converted using such methods would be considered individually, but that no further such usage of those type of NFA registered conversion parts or kits would be allowed. In effect, anyone holding a NFA registered machine gun conversion part or kit, that required receiver modifications to install it but had not already performed such an installation, were now in possession of contraband. This ruling was quickly applied universally to other conversions of semi-auto guns that had had their unmodified Title I receivers NFA registered as “machine guns”, but had been mechanically converted to automatic fire by the use of unregistered conversion parts, a practice heretofore having been declared marginally acceptable. The H&K conversion story all of a sudden got considerably less clear at this point. According to ATF, what now existed with H&K type NFA conversion guns were several disparate legally defined classes of registered conversions, though ostensibly all on the same type of gun!

 So...just what ARE you looking at? The many possible scenarios.

First, you had those Registered Receiver conversions that had been originally mechanically converted to a legally defined machine gun receiver employing a factory MG style push-pin, swing-down style trigger housing. These guns occupy the pinnacle position of NFA conversions desirability status due to their unquestionable registration legality, duplication of factory mechanical features and cosmetic appearance, and ability to allow owner changes in factory style trigger groups along with enhanced spare parts availability.


Three styles of selective fire packs. 

On top is a converted pack using a registered sear, the
 center unit is a converted original factory machine gun
 pack adapted to fit a clip-on mounting.  The bottom is an
 unaltered original SEF pack for push-pin mounting.


Next up, those “Registered Receiver” guns that had the unmodified Semi-auto Title I configured receiver NFA registered as the “machine gun”, but had been originally converted with an unregistered conversion part or device. These guns have several handicaps and vices. First, because of the above mentioned legal rulings their receivers could never be further altered to a push-pin, swing-down trigger group mounting style, which committed them to forever using the same trigger group and fire control pack as had originally been installed on the date of initial conversion and NFA registration, with one minor possible exception. Because of the usage of that unregistered conversion part in the modified semi-automatic fire control pack, that conversion part itself could never be replaced, only repaired! This is legally prevented because to replace it with a new one, if damaged or lost, would constitute new manufacture of an unregistered machine gun which was made illegal by the new 1986 Laws. Also, this prevented easy replacement of trigger group spare parts, as to convert the semi-automatic fire control pack to replicate selective fire, many of the fire control parts had to be heavily modified themselves. This was due to an altered mechanical geometry required by the need to have the same mechanical functioning as a factory selective fire pack, but in a geometry necessitated by the requirement of having the lower front edge of the fire control pack adapted to mount on the “clip-on ledge” of the semi-auto receiver. So, to get suitable replacement or spare fire control parts, one has to have standard parts that were specifically altered for usage in such a conversion, which are not typically available as easily as factory MG parts are now. The key is that with these types of “Registered Receiver” conversions, the NFA registered “machine gun” is the original combination of the unaltered clip-on style receiver, and the originally installed unregistered conversion part...those two entities can never legally separate, they are  “married” for the life of the weapon!! All of these reasons are why this type of  “Registered Receiver” conversion occupies the lowest level of desirability amongst registered receiver conversions, and are priced accordingly.

Even more disheartening in present times is the fact that since conversions of this type use an unregistered conversion part of most likely a proprietary design, unless you can re-install that exact conversion part in a different pack, you cannot replace the original style fire control pack with a perhaps more desirable version such a “Navy Group”, or 2-or-3-shot “Burst” style ambidextrous lower group!! (This problem also effects conversions using a “NFA Registered Sear” as will be seen)

[LEGAL WARNING! While it may be physically possible to adapt a factory select-fire ambidextrous style “Navy Group”, or “Burst” type trigger pack and plastic housing to fit a clip-on semi-auto receiver, it is patently ILLEGAL to simply adapt such a trigger group to fit, and then just clip it onto a “Registered Receiver” gun using a clip-on style housing attachment as you have just made a new, unregistered machine gun by doing so you’ve just created a complete conversion device to convert a semi-auto receiver!! The legal issue here, again, is that you are left with the original, unregistered conversion part being freed up when you swapped them out on your Registered Receiver conversion. There are currently available many of these adapted “Navy”/”burst”/Ambidextrous trigger groups offered for sale, but they are ALL illegal, unregistered conversion devices!! The ONLY way they are BATF legal is if they have been NFA registered as a Conversion device! This had to be either before the May 19, 1986 cut-off (which is HIGHLY unlikely due to very limited availability of these styles of factory parts then), or as a “post-86 Dealer Sample Conversion device”, or made at the request of Law Enforcement Agencies.]

Lastly, somewhere in between the above two more common versions of registered receiver conversions are a very select group of Title I guns that were converted to select fire by a more questionable method...at least according to ATF Technology Branch after their early 1988 determinations and rulings. As alluded to previously, a select few semi-auto guns are known to have been converted by having their unmodified clip-on style receivers NFA registered as a “machine gun”, and then proceeding to attach a normal factory original MG select-fire, fire control pack and housing that had both been suitably modified together to clip-on attachment, as described above. These guns most likely were never visually inspected by ATF Technology Branch for suitability and conformance to approved methods prior to registration, and therefore “sneaked by” and into the NFRTR. In any era, these style conversions would be suspect as they contain an unmodified Title I receiver with its clip-on trigger group attachment point, and an unregistered drop-on conversion assembly. These guns are ripe for ATF making a determination that they were improperly registered, and may be subject to reclassification. Why the original registrant did not originally just register the converted trigger group assembly alone, instead of the receiver of the Title I gun, is a mystery...and a lingering problem for the current owner. These guns are just too much in a legal grey area to attach a reasonable sense of value to, and they are the perfect example of why a prospective purchaser of a “registered receiver” H&K conversion gun needs to extremely vigilant and aware of just exactly what particular gun he is looking at.

Within this understanding lies the major financial difference in perceived value of “registered receiver” conversion guns. A ‘fully converted’ gun able to use factory push-pin style, swing-down trigger groups is always deemed to be more inherently ‘valuable’ then a gun using a ‘clip-on’ trigger group, though the mechanical functioning is identical between the two guns. The perception is two-fold. First, on receivers able to use swing-down groups the ability to choose between the various factory offered styles of fire-control set-ups is greater, and they are simpler to exchange. Secondly, on registered receiver guns that employ a clip-on style group, at least one of the fire-control parts will be an unregistered conversion part, which means in practical terms that should this part break or be damaged it can only be repaired and never replaced, which limits to some small extent its serviceability. Thus, the ‘spare-parts question’ of a gun using a registered receiver employing a swing-down style trigger group is considerably better; hence an appreciable rise in value and pricing is accorded.  




Above, an extremely rare 0-1-F-3 style burst group with burst mechanism located
 in the rear of the pack.  These are the only burst packs that can mount an unaltered
 registered sear. These groups command a substantial price.  Below, the inside view
 of the 0-1-F-3 burst pack showing a radically different mechanical arrangement
 compared with the more common "ambidextrous" style burst packs.


It is easy to see then that a ‘Registered Receiver Conversion’ is not so simply defined as just a receiver converted to be functionally identical to a factory original machine gun receiver with a push-pin style, swing-down lower. It can exist in different forms, and those different forms determine a large difference in a monetary value, user friendliness, and perceived desirability.

 The “Full” Registered Receiver Conversion...Top to Bottom

 As previewed earlier the most desirable conversion method was to completely restore the Semi-auto Sporter receiver configuration back into a factory machine gun specification. This actually was done by several established shops and some individuals with the proper know how and skills, when they had the factory original select-fire trigger packs to install as part of the job. In order for this type of conversion to be unquestionably legal it had to have, at the time of initial conversion, the dedicated semi-auto receiver features completely removed, and then the two distinctive factory original machine gun receiver features restored. In proper order, this meant milling off the semi-auto receiver clip-on ledge, then align drilling the two pivot pin holes, and finally, welding in the pivot pin bushing, ALL THREE STEPS!

In actual practice, this was done with varying degrees of success, and/or completeness. One of the more well known Class 2 manufacturers had the habit of roughly milling off the clip-on ledge and drilling two holes, but not installing the pivot pin bushing which leads to oblongation of the pivot pin receiver holes after time. Many times the location for the pivot pin holes was roughly estimated by placing a factory style swing down lower on the receiver and using it as a template for drilling the pivot holes, which obviously led to a great deal of inaccuracy in their locations, all of which should be verified if a particular gun displays any untoward habits. Those conversions done by competent machinists using factory drawings and/or production data are unquestionably more desirable. Additionally, if a factory MG style “flapper” magazine release assembly was to be installed, the proper alignment of the pivot pin holes and the relative alignment of the pin bushing became critical, as this is what the flapper sleeve rides on, and bears against. An improperly aligned pivot bushing can prevent proper magazine release functioning.

Once the conversion of the trigger housing attachment was completed the only other significant detail to proper selective fire mechanical function was the bolt carrier. All HK type bolt carriers are manufactured the same, to machine gun specifications, until they are destined for a particular product line. For use in a semi-auto Sporter gun, the under surface of the carrier which contains a milled contour to activate the automatic sear trip lever in machine guns, is milled straight through, rendering it incapable of use in a machine gun.  A restoration of this carefully contoured tripping surface is required. The bolt carrier is manufactured from high-tensile strength alloy steel which has been heat treated to resist wear, and on some versions additionally surface hardened in critical wear regions. This high quality material content and manufacturing method resulted in a piece that was extremely difficult to modify or alter with ease. To do it properly, the carrier should have been fully annealed, then had the milled areas filled in with a similar alloy content weld or plasma fill, finish contoured to factory surface spec and then final heat treated, and/or surface hardened, to restore the original wear and longevity properties. Almost no one ever did this, as it is complex and time consuming, not to mention costly. Usually a weld fill and regrind sufficed, although a couple shops adopted Stellite welding in an attempt to add wear resistance back into the tripping surface(s). The modified bolt carrier is one of the most commonly encountered areas of trouble from the original conversions, and fortunately now, a simple solution is at hand. With the current easy availability of original factory produced machine gun parts and parts sets, the best solution is to install a factory machine gun style bolt carrier, which has the proper sear trip surface intact.

With these modifications complete, the guns’ receiver has been fully restored into a virtual clone of the factory produced versions. As can be immediately seen, a gun so restored has the advantage of using the full range of “off the shelf” factory MG accessory trigger groups and spare parts. With the current ease of obtaining the more “modern” styles of plastic housing  “Ambidextrous” trigger groups such as the so called “Navy Group”, or the various “Burst” style groups, a “Complete Conversion” is often valued an order of magnitude higher than the unmodified “Registered Receiver” conversions due simply to its unique ability to utilize all factory parts and accessories as originally intended without fitting or alterations. Essentially, these guns beat the MG Import Restrictions contained within the Gun Control Act of ‘68, and at a considerably lower cost than a true “Original” which is undoubtedly now considered a collectors’ piece.

Second Choice: Conversion “Parts” and “Devices”...Registered and Unregistered!!

 As we have seen above, unless the receiver of the gun was restored into full machine gun configuration, the mechanical alterations needed to produce automatic fire were necessarily contained within the fire-control group and these modifications were usually performed in a similar manner, whether or not the conversion part or device was NFA registered or unregistered in and of itself. In the same fashion as the “Registered Receiver” categorizations, “Conversion Parts or Devices” can be divided up into two distinct sub-categories. Firstly, the mechanical conversion may be accomplished by use of a single conversion device, such as a conversion SEAR, or secondly, by alteration of the whole fire control group as a unitized assembly, such as a converted factory select-fire pack.

There is necessarily some cross-over and blurring of the lines of distinction in discussing these items as they all function absolutely identically, the only difference being with which basic components you started, and in which direction you preceded to arrive at the same mechanical destination. Conversion partssuch as sears are necessarily part of conversion groups (devices), they are just discussed as a separate component part as opposed to the unitized assemblies of converted factory original groups, because of the needed mechanical alterations to install them into functional automatic-fire status in a semi-auto fire control group into which they are being adapted.

First off...Registered Conversion Groups.

This category can be loosely described as either “Converted Semi-Automatic only fire control groups”, or “Converted factory original Selective Fire control groups” Each method has its advantages and limitations.

The equivalent to the  “Full Registered Receiver Restoration” in terms of mechanical purity and factory feature replication would be what is known as a “converted Selective-Fire Trigger Group”. As the name implies, this is nothing more than a stock factory produced select-fire trigger group suitably modified to mount onto an unmodified clip-on style Title I receiver. These are almost always found to be an original metal grip framed “S-E-F” style housing, and an NFA registered item as a conversion DEVICE, in and of themselves, in order to not be in any legally grey area. Although as mentioned above a few may be found mounted on an NFA Registered, but unmodified, clip-on style receiver as an unregistered conversion device. All internal mechanical components are factory select-fire design and this greatly enhances the spare parts scenario, as there are no modifications done to any of the trigger group functional parts.

When we discuss this specific type of conversion device, we are considering only the actual mechanical pieces required to adapt the complete group as an assembly onto an unmodified Title I receiver mounting. This is accomplished by first adapting the factory select-fire sheet metal trigger pack frame to mount via a clip-on style. Then by adapting the outer frame grip housing to allow for this new mounting method by relieving the lower frontal area of the housing “box” to permit the receiver ledge to protrude through, and finally by providing a horizontal mounting point for the receiver ledge to hold fast to. This adaptation will also be seen when we examine the installation of newer style plastic grip frame housings to clip-on style receivers when employing registered “auto sears” in converted semi-auto trigger packs. Or in rarer cases, when an owner desires his converted semi-auto trigger pack utilizing an unregistered conversion part to be reinstalled in one of these newer housings. There are significant limitations to this in legal terms though as discussed above. The NFA registration reflects the status that the ENTIRE unit is the registered item as it requires extensive modifications to both the grip frame and the enclosed trigger pack to adapt them to fit and function on a clip-on style Title I receiver. These two individual components cannot ever be separated legally, as again, it is the combination which was originally registered as the “device”. What this means is that exchanging the metal “S-E-F” outer grip frame housing for a newer style plastic “S-E-F” grip frame housing is not allowed if the metal housing was part of a registered combination. Most of these units were marked on the metal housing exterior with the required identifying markings when registered.

The real intrinsic value of owning one of these converted, adapted, and NFA registered factory select-fire trigger groups is obvious in that it is, in a mechanical sense, it is absolutely genuine and identical to those trigger groups that came on factory machine guns, only the mounting method has been altered. The internal mechanical components are fully interchangeable, and more importantly, they are situated in the normal design positions providing the intended mechanical geometry of function. The designed in mechanical geometry remains fully unaltered which will retain the designed cyclic rate-of-fire, as on a factory original machine gun. This is a most significant point to be understood, as this mechanical geometry is not the same in conversions done by altering a semi-auto trigger pack to function in the selective fire mode using a “Conversion part”, or “registered auto sear”. This last statement directly leads us into the most commonly misunderstood arena of the whole “H&K Conversion” dilemma...

‘Sears’, ‘Trips’, and‘Catches’...the Pandora’s Box of Conversion ‘Parts’

 When HK GmbH set about to redesign the original select-fire control group to mechanically remove the automatic or cyclic fire mode, they chose to alter one of the original safety features built in to the design. Like the FN FAL mechanics, the “automatic sear” is also a form of safety sear designed to enhance the mechanical safety by preventing hammer release and firing pin contact before bolt lock-up in the final battery position. Removing the automatic sear removed one level of mechanical safety, as it is always functional whether or not the fire control selector is in the cyclic fire mode. Selection of cyclic fire mode merely allows the trigger to drop the “secondary” sear (also functioning as the disconnector) backward and down out of contact with the hammer engagement surface.

To adapt the currently produced parts with as little change as possible to a Semi-automatic only configuration was quite easily done from an Engineering standpoint. This adaptation was accomplished with a minimal redesign of the three elements of the fire control group. The main difference between the two fire control options lay in the two independent safety systems. Since these two independent functions were arranged as separate mechanical pieces, they only had to remove the automatic sear (‘catch’) and its attendant release lever from the fire control pack to eliminate the option of cyclic fire. With a small modification to prevent the selector lever from rotating all the way down into the cyclic fire mode position the trigger pack was now arranged to offer only two positions: “Safe” and “Fire”(semi-auto only). The secondary sear (‘disconnector’) remained as always, merely prevented from dropping down out the way of the hammers travel, thereby always forcing it to catch the hammer, “disconnecting” it from the trigger for normal semi-automatic operation. On HK GmbH produced semi-automatic guns, the primary sear notch on the hammer was also ground off to prevent easy re-conversions. On some of the licensed produced HK91 clones the hammers are not fully adapted to semi-automatic only operation. The selector levers are identical between select-fire and semi-auto only versions.

Combined with the obvious redesign of the relevant fire control parts to eliminate the selective fire capability and restrict it to only semi-automatic fire, the actual sheet metal box frame that holds these components was modified into a semi-auto-only configuration. With the deletion of the automatic sear and its pivot pin position, the select fire trigger pack frame could be altered to mount on the newly designed clip-on receiver mounting ledge. This was accomplished by cutting a distinctive ‘notch’ in the lower frontal area of the sheet metal box, the area formerly occupied by the sear pivot pin holes. Importantly, one other major mechanical item was connected to this location. The trigger (and auto sear) return spring was mounted inside the automatic sear arms and was held in place by what is known as a ‘distance sleeve’, which is nothing more than a hollow tube contoured to hold the spring in position and it slips around the sear pivot pin. This single fact is the least understood problem of conversion packs using the clip-on mounting style.

Due to the deletion of the full-automatic sear position in the semi-auto pack, necessitated by the new front clip-on mounting ‘notch’, the pivot point for the trigger return spring had to be relocated to a more rearward position in the pack frame than in the selective fire frame. This relocation is the major mechanical distinction of the “registered/unregistered auto sear” conversion.  The “conversion sear” is forced to use this new trigger return spring pivot pin location as the mounting point for the newly installed conversion auto sear which places the conversion sear in closer physical proximity to the sear contact bearing surface of the hammer. The trigger return spring used in semi-auto packs is also formed differently than the selective fire version. This relocated spring must also now function on the conversion sear in the same manner as the factory select-fire unit, but it is forced into a differing contact geometry and so acts accordingly. Whether NFA Registered or not, all these “conversion sears” mount and function the same!  The only significant difference is found to be whether or not the original factory release lever is still employed in the conversion. In some designs of conversion “sears” the release lever is integrated into the sear design, so eliminating one extra part. And whether or not the conversion used a modified originally semi-auto only pack frame or an adapted, cut and blocked, originally select-fire pack frame, the “conversion” geometries of the newly installed mechanics are the same!

 Conversion Sears…It’s all in the Timing!

 As we have discussed above, the lower frontal area used to clip-on the front mounting ledge on a semi-automatic fire control pack is where the factory original select-fire, fire control pack mounts the pivot pin holes for the automatic release sear. Since the auto sear pivot pin is also where the trigger return spring is mounted, HK GmbH in its redesign to semi-automatic-only operation selected a new location for the trigger return spring. It was now riding on a pivot pin used only to mount the return spring and its redesigned semi-auto only distance sleeve. This new return spring location in the semi-auto only pack is the location that conversion sears are mounted in, the same effectual location as in a factory original select fire pack. NFA Conversion fire control packs, either original semi-auto or modified select-fire versions, employing either a registered or unregistered conversion sear, utilize that relocated trigger return spring pivot pin hole location. That relocated pivot pin position for the new conversion sear has a very important effect on mechanical functioning.

On original factory select-fire packs the auto sear pivot pin holes are located 7mm farther forward in relation to the trigger pin pivot hole, and 3mm lower than in conversion packs. This rearward relocation of the sear engagement to hammer contact surface in conversion packs has the net effect of causing the hammer to be held in a different vertical position than on a factory select-fire pack.  With the hammer cocked and held by the two versions of auto sear (‘catch’), there is a large difference in allowable hammer travel upon auto sear release. On a converted pack the hammer rides 12mm higher than in a factory select-fire original pack. This difference in hammer travel to firing pin contact is equivalent to lock time; a decrease in lock time will cause an increase in cyclic rate! This is what is commonly known as the “timing” of the conversion, and proper timing is critical to more than just mimicking factory cyclic rates. Remember, the original factory design uses the auto sear also as an effectual safety sear, preventing inadvertent hammer release until final bolt battery lock up. Since the gun employs a delayed blow back action using roller locking, the only thing preventing a totally unlocked cartridge ignition is if the bolt rollers have entered their locking recesses. This is theoretically only possible if the locking piece can travel to full extension by forcing the rollers outward allowing the firing pin tip to protrude out of the bolt face. However, in certain situations the locking piece can forced under hammer spring pressure to contact the firing pin before complete extension of the bolt rollers, particularly in a gun with worn components such as the locking recesses in the barrel extension. As such it is critical that the auto sear release occurs only after complete extension of the bolt locking rollers into their locking recesses. This timing is particularly effected by two things: firstly, the release lever camming trip surface on the underside of the bolt carrier, secondly by the hammer-sear contact geometry. The release lever camming timing is merely a matter of restoring the original factory bolt carrier underside contours, something that was usually done by most manufacturers of these conversions. Restoring the hammer-sear contact interface geometry is considerably more difficult to achieve. Due to the relocation of the new conversion auto sear, if an unaltered factory original hammer is employed the new conversion sear will hold the hammer in the higher position as described above. This is an induced reduction in lock time, and if combined with an improperly re-contoured bolt carrier tripping surface, it may cause early release of the hammer as opposed to factory specifications. If this occurs in a well worn gun with worn locking recesses the bolt rollers may not be fully extended when the hammer spring forces the locking piece forward to contact the firing pin. For this reason a properly converted hammer used in a “sear conversion” will have had the factory original sear notches cut on the front lower face of the hammers welded up and the re-cut to allow for the new geometry of the relocated conversion sear. Unfortunately, not all “sear conversions” have had this accomplished. While the safety reasons as explained above should be enough, the most common reason desired to have the hammer re-contoured is the restoration of the factory specified cyclic rate for the particular gun model in question. Even a minor increase in cyclic rate will cause a large increase in recoil forces and induced wear and tear, something to consider since these guns are often extremely expensive now.

(Note: there have been observed slight variations in these actual geometries depending upon the individual conversion in question. These measurements are representative of industry accepted practices for alterations required to adapt these conversions, individual guns may exhibit minor variations that should not be considered a defect if the gun runs acceptably. An insignificant variation in cyclic rate is the usual net effect)

There are a few more minor modifications required to get these conversion sears to properly function. The original disconnector (“sear” in HK parlance) must be slightly relieved to allow it to properly function in the new mechanical arrangement. A “sear conversion” converted pack uses a factory original machine gun distance sleeve, as the conversion sears occupy essentially the same dimensions as the factory original ‘catch’, and so the longer semi-auto only distance sleeve cannot be used. The semi-auto only and factory select fire trigger (and auto sear) return springs are also different. The auto sear (or ‘catch’) release lever is almost always used in its factory original form, and location, the design of the NFA conversion sear accommodating this arrangement.

Conversion Sears:  Equivalent, but often different

We have been discussing those NFA conversions that retain an unmodified Title I style receiver and attain select fire operation by utilizing a conversion “part” known almost universally as a “conversion sear” in nearly generic terms. There were only a relative few proprietary designs of these conversion sears, yet there are some important distinctions to be made.

As discussed, NFA conversion sears may be divided into two main categories, those that exist as a direct replacement for the factory designed original ‘catch’ and are designed to function in the exact same manner if in a slightly different geometry by using a factory type release lever. And secondly, there are those styles that are designed to replicate the factory mechanics by combining the functions of the factory ‘catch’ and the release lever in one conversion part. Again, depending upon the individual Class II Manufacturers’ or Qualified Individuals’ personal choice at the time of conversion, or the timing of the registration involved, these conversion parts may or may not be individually registered in the NFRTR as “machine guns” in and of themselves!! They do however all function identically and may exist in exactly the same physical form with some being NFA registered themselves, and some being installed onto gun receivers that are the registered item as described above.

The first kind of conversion sear is designed to directly replace the factory ‘catch’, or auto sear. Despite the method or materials of manufacture they are identical in all significant mechanical aspects. The factory ‘catches’ were initially designed to function with a curved end “finger” of the upper arm extension acting against the straight undersurface of the release lever by riding in a cammed arc in direct friction contact. As such they became known as “friction catches”. This, as opposed to an updated version which employed a ‘catch’ that was designed with a small roller wheel at the extreme end of the extension arm, acting against a curved underside of a redesigned release lever arm, these are known as “roller catches”.  All NFA conversion sears of the direct replacement type were made in the “friction catch” style!!  

Direct replacement type conversion sears were made of steel and by casting, stamping, or machining of bar stock depending upon individual choice or manufacturing ability at the time. Quality and durability vary, and though all are serviceable there are differences. Several physical styles of these “direct replacement” sears were created. Some are dimensioned and shaped very differently than the factory friction ‘catch’ they are designed to replace, and as such they may require proprietary changes to the other trigger group parts that are slightly different than what would be considered “industry standard” as described above.

The “most desirable” types are constantly being argued, but if replication of factory design, dimensioning, and materials are considered there is one clear winner: the stamped and formed conversion sears are hands down closest to a factory friction ‘catch’, and are regarded as such. Despite construction type and materials the single most important factor in durability of any of these conversion sears is proper heat treatment. It is known that some of the machined or cast sears were not heat treated correctly, if at all, and they will exhibit excessive wear profiles. Most of these are known to be installed as unregistered conversion sears residing in conversion packs mounted to receivers that are themselves NFA registered.  Fortunately, any of these incorrectly heat treated sears can be re-heat treated by a competent and knowledgeable Gunsmith. Additionally, the upper arm extension finger can exhibit excessive wear, galling, or cracking. The only other problem to watch for is that some of the cast sears have been found that have improperly located pivot pin holes, and these off-axis pin holes can be worn out of round over time. This too can be fixed rather easily. Those stamped and formed conversion sears produced in a tandem manufacturing effort by S&H Arms and Fleming Firearms, and the cast sears by S&H Arms are regarded as the most likely to offer effectual service and lasting value.

When moving into the realm of the combined ‘catch’ with release lever type of “conversion sear” we find a much broader interpretation of what constitutes a “conversion part” as mechanically and legally defined. These are generally speaking found to be of the earliest era of NFA conversions and exist in physical forms and dimensions that are generally totally different than a factory part. As described earlier, many Class II Manufacturers were forced to modify the existing semi-auto-only parts that came in the to-be-converted gun because of the total lack of available factory select fire parts then available on the open market. Accordingly many of these converted semi-auto parts resemble Modern Art masterpieces!! At least one competent Class II manufacturer, Jonathan Arthur Ceiner, Inc., essentially replicated the factory combined ‘catch’ and release lever as found employed in the HK21E/21A1 LMG into a proprietary design that is exceptionally well done.  It is generally found though, that most of these combined function conversion parts are far from an ideal set-up. It is impossible to offer much serious advice separate from the general advice given above, as there are just too many varying styles installed by manufacturers with widely varying mechanical competencies. Except as stated above for the singular exception for Ceiner conversion sears, these combined function sears are regarded as being the lowest value in terms of desirability.

Through the Glass, Darkly. 

With these NFA conversions employing a gun receiver with the Title I style clip-on style trigger group attachment point there remains a very tempting trap awaiting those whom would knowingly or unknowingly slip into it. Why this is may not be readily apparent, and even then there remains some stubbornness in accepting the legalities of the current situation regarding this. The desire to update their gun with a newer style plastic “Ambidextrous” or “2-shot/3-shot Burst” style trigger group remains almost universally strong among current owners, however there needs to be issued one final warning regarding doing this.

The ONLY LEGAL WAY to install a newer style “Ambidextrous” or “2-shot/3-shot Burst” style trigger group is if the original, registered or unregistered, conversion sear (or “catch”) can be mated to function in the “Ambidextrous” or “Burst” trigger pack mechanics. This means you must discard the factory “sear (‘catch’)” that came in the new pack and affect installation of the original registered or unregistered conversion “sear” that was utilized in the original fire-control pack. Some few proprietary “sear” designs can be adapted to fit and made to properly function, most can’t.

As detailed earlier, the conversion pack frame itself has to have been altered to fit the clip-on semi-auto receiver, while having the new style plastic housing adapted to fit the clip-on receiver, and then “blocked” to prevent re-installation of an original select-fire trigger pack. Now often enough this is done with the newer style plastic S-E-F housings which are essentially identical in function to the older style metal S-E-F versions, and this poses no legal problems by doing so. Because most of the “direct replacement” style conversion sears are modeled after the friction sear found in the factory original metal or plastic housing style S-E-F trigger groups, adapting just the plastic housing presents no mechanical problems.

The legal and mechanical problems arise when we are trying to adapt a newer style “Ambidextrous” or “2-shot/3-shot Burst” style trigger group. And there is a very good reason not to simply go ahead and have such an adaptation performed by simply discarding the originally installed conversion pack and clipping on a newly adapted one. The mechanics of the Ambidextrous or burst trigger groups are VERY different than the standard S-E-F type of fire control pack, with many of the component parts totally redesigned to properly function with the burst counter clockwork mechanism installed. Ambidextrous style trigger groups only offering selective, but not burst, fire modes are also arranged identically and so suffer this same problem. H&K GmbH actually offered two very distinct styles of “burst” mechanisms. The first version externally resembled the S-E-F type of housing and located the burst clockwork mechanism in the rear lower portion of the pack frame.  This is significant as it left the front ‘catch’ position identical to the S-E-F fire control pack!! This means that this style burst pack, and only this style, can be readily adapted to function with those conversion sears resembling externally the factory friction catch!! These very rare fire control packs command an extreme premium in today’s market as they can be easily adapted to function with the most common versions of conversion sears, and the market for using them to install registeredconversion sears primarily, is very strong. The second style of burst pack located the clockwork mechanism more toward the frontal region of the pack frame and this forced a complete redesign of the internal components. These new style burst packs cannot be easily or readily converted to use a NFA conversion sear!! Many styles of proprietary NFA conversion sears simply cannot be adapted to function without major mechanical redesign. Due to this fact, there is a large temptation to avert the hassles in doing so by simply adapting the entire trigger group and exchanging it on a “Registered Receiver” guns’ receiver that formerly used an unregistered conversion sear. Attempting to do so leaves one extremely vulnerable in a legal sense¼.

One must remember that when doing semi-auto conversions was a lawful enterprise, there were not that many shops actually performing these jobs, and the vast majority of guns converted were done by a relative few. It became quickly known which shops preferred to do which manner of conversions, and by what methods they commonly converted these guns, so a savvy ATF Field Agent can easily tell if a particular gun should have come with a particular type trigger housing, or fire control pack, etc. Also, the easy availability of the “new style” Ambidextrous/Burst trigger groups today was not the case even a few short years ago, so the likelihood of one having been originally installed is not too great. BE FOREWARNED!! And be fully prepared to PROVE that your “Navy Group”, or “Burst” trigger group now residing on your clip-on style “Registered Receiver” conversion gun contains, and has been adapted to function with, the original unregistered conversion part that the gun came with...and yes, it HAS happened.

“Losing” the original fire control pack, and its original unregistered conversion part, for a newer Ambidextrous/Burst style trigger group is a good way to lose the whole expensive gun...and possibly more.

So...Now you know!

Hopefully now, there is a clearer understanding of just how complex the topic of NFA Conversions on H&K style weapons is! It is a fact that since these guns represent one of most modern and desirable types of Civilian legal machine guns available, many enterprising Class II manufacturers sought to do conversions on these guns. Unfortunately, all of these attempts were not created equally. Therefore, it is paramount to assess the basic conversion type in question before a fair market value can be assigned in this quickly escalating market place of recent years.

            (All content Copyright 1999 by Thomas T. Hoel)


[Home] [Price Guides] [Subguns] [HK NFA Conv]

Copyright © 2006-2023 PJW - All rights reserved                        [Sitemap]                                       Comments to topgun@machinegunpriceguide.com