Mechanics of the H&K NFA Conversions: Part III

By Thomas Hoel

Tom Hoel has done an outstanding job of his latest chosen assignment, classifying and technically describing the mechanical differences and roots of the various HK machine guns in the US Market. The work was so extensive that we are running it in a three part series. This is not a definitive work on “Who made what gun”, but a targeted technical work that should enable the prospective buyer to understand “How”, “Why” and perhaps more importantly; “What are the features that I want from the HK I am buying”. In this the final part, Tom continues into the mechanics of the registered HK “Sear” conversions - Dan

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

As we have seen in Part II of this series, 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 parts such 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.

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 hammer’s 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 it’s 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 Qualified Mfg, 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 registered conversion 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 an 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 sense?.

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.

Photos for this three part series were provided by James O. Bardewll, Esq. and Dr. Phillip H. Dater, M.D.

This article first appeared in Small Arms Review V3N9 (June 2000)
and was posted online on May 1, 2015


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