UZI Talk - UZI Conversions (page 3)

UZI Conversions - Page 3

The Three Conversion Methods

Now, with an understanding of the various mechanical differences between the SMG and the semi-auto carbine, it is relevant to discuss the actual mechanical methods of conversion that will be found on a NFA registered UZI carbine SMG conversion.

It did not take long after Action Arms Inc. began importing the first carbines to the USA in 1980 before some very industrious minds began to figure out how to convert the gun into a full-auto firing (versions in full-auto only, and selective full-auto fire) submachine gun.

The first type of conversion took clever advantage of the design of the striker-fired, closed-bolt, semi-auto mechanism in spite of all that IMI had done to prevent easy conversion. In fact, some of the IMI changes actually facilitated this type of conversion! Recall from above, that the first Model A guns to come in were slightly different than all those that followed. IMI had forgotten a few very important details it seems! For the open-bolt mode of fire, the full-circumference rim on the bolt face, designed to guide the cartridge into the chamber prior to the fixed firing pin striking the primer cap, is critical. Without this lower rim on the bolt face the cartridge bottom rim will hang up on the firing pin, causing jamming as it enters the chamber. The cartridge rim will climb into the bolt face too soon. On a closed-bolt weapon there is no firing pin in the way, until sear release of the striker after bolt closure and chambering, so the elimination of that lower lip is of no consequence unless somehow, that fixed firing pin, or its equivalent, suddenly reappears! That is exactly what those industrious minds figured out how to do. If the striker could be attached to the rear of the bolt it would act as an open bolt, with a fixed firing pin protruding at the bolt face. Even simpler, it was noticed that the striker spring was strong enough to carry the striker in place against the rear of the bolt during chambering, enough to cause reliable primer ignition! Now you were half way there. The earliest Model A bolts had the full-circumference cartridge holding rim already, remember? Two thirds of the way there. If you could somehow allow the selector lever (early Model A select levers had three detent notches) to move forward far enough to interrupt the disconnector function, the bolt would continue to reciprocate until trigger release.... full-auto (only)! Method one done.

In finer detail, these were mostly all registered receiver conversions for two basic reasons. Reason one, because at this time there was no economic reason NOT to register the whole gun as an SMG, and reason two, the ATF rulings which brought about the so-called "machine gun conversion parts" distinction was just beginning to get started with the “AR15 drop-in auto sear” mania and the like, with the UZI conversion scene adding to the fire for the following reason. From the just discussed conversion method above, it becomes apparent that one could convert a semi-auto carbine to fall-auto with no receiver modifications. If a new selector lever that bypassed the disconnector function could be made that could drop-in, and replace the semi-auto one directly. That was exactly what was done in some cases, with a "Conversion Selector Lever" being NFA registered as the machine gun itself, and either installed in a gun by a Class II Manufacturer or owner (on Form 1), or sold by itself. ATF promptly ruled this lever was a machine gun with the usual warning against unregistered possession, etc.

So, early Model As {with the full SMG style lower bolt face rim) can be found with this NFA registered lever as the only difference, but they will be able to fire fall-auto only, not selectively. For later bolts with the lower rim machined off, it was necessary to reinstall that rim as in an SMG bolt face. This was usually done by welding a machine contoured ‘half rim ring’ back onto the bolt face itself. For the selector lever itself to NOT be considered a registerable part under the NFA, it had to be of the original design with the right-angle finger attached that engaged the disconnector (and blocking piece welded onto the front lower shelf of the trigger grip frame housing). Early Model A select levers with all three detent positions could be used just as a regular SMG part if the blocking piece could be removed, but doing so altered the ‘as manufactured’ status of the gun, and was disallowed by ATF, unless the receiver was NFA registered, and this was the only way to remain legal with a conversion of this type early on. The proper conversion method then had the blocking piece removed to replicate the SMG control housing function. Later semi-auto select levers were devoid of the third detent notch, so that had it to be reconfigured on later guns undergoing conversion. This describes the various NFA registered guns that can be encountered from the earliest conversions done. This phase lasted only about a year or so, and changed very quickly when those industrious minds went to work again, this time on the bolt itself.

Since to modify the relieved bolt face seen on most Model A guns required accurate welding and reheat treating of the bolt, it seemed easier to start with a bolt already manufactured in the full SMG configuration as regards open bolt firing requirements. If you started with a SMG type bolt the only major modification needed would be to allow placement inside the semi receiver with its blocking rail. A simple job of milling a slot in the correct location to mimic the design of the semi-auto bolt, and the gun would function in the open bolt mode. And so the now famous "slotted bolt" was born. The first versions sold openly were designed to accommodate the normal closed bolt recoil spring and buffer arrangement which was slightly longer in depth on the semi gun because the semi closed bolt did not need to reciprocate as far back into the receiver because of the striker assembly riding behind it. To this end, the cut track opening for the cocking slide in the top cover had a slightly shorter track, as the cocking slide did not need to be drawn as far back with the striker engaging the sear instead of the bolt. To retain as much of the original SMG operating characteristics as possible, it was soon seen that by replacing the semi recoil and buffer assemblies with a standard SMG version the slotted bolt could attain all the relevant dimensions of the SMG, save for the milled slot in the side. By increasing the length of the slotted conversion bolt to normal SMG spec it also restored the normal cyclic rate by virtue of the increased mass and recoil travel. The only down side to this bolt, and all future NFA registered conversion bolts of this description, is that it now required the full travel of the cocking slide to engage the bolt notch with the sear while cocking the weapon, such that it became necessary to now mill open the length of the cocking track opening to normal SMG specs too. It was often easier to just replace the whole top cover, with the added benefit of the SMG ratcheting mechanism coming along for free.

This all came to an immediate halt in mid 1982, when ATF ruled that such fixed firing pin, slotted bolts, like the drop-in selector or lever, were machine guns in and of themselves, so future manufacture, sale, and possession was to be in full compliance with the NFA. There were though a small number of legal registered conversions made by using these unrestricted, restricted, conversion bolts prior to that ruling, and they were allowed (grandfathered), as they were previously lawfully, registered in full compliance with the existing laws, though the conversion bolt now needed to be married to that particular serial numbered receiver. If it ever needed replacement you were out of luck. With this restriction some of these registered receiver guns simply ditched (read: destroyed) the UN-registered conversion bolt and went ahead and finished the receiver modifications that allowed utilization of an original, solid SMG open-bolt, as these were the only bolts that remained legal for unrestricted sale and transfer after the ruling.

This is the basis then for the second most common method of conversion, that of using an NFA registered, slotted conversion bolt, in an otherwise unmodified Title 1 semi-auto receiver. These slotted conversion bolts were still made and marketed, but they now had to be individually NFA registered, and a lot of them were! The slotted conversion bolt is operationally identical to the standard SMG open bolt, except for the milled slot in the right upper side face. All other relevant dimensions and operating characteristics are the same, with one small nod to the inescapable reality that it was to be used in an unmodified semi-auto carbine receiver. All semi-auto carbines incorporate the barrel modifications as described above such that a normal SMG bolt face, if used in an unmodified semi-auto receiver, would not properly close against the rear of the barrel chamber, as the restrictor ring which surrounds the rear of the semi-auto barrel would interfere. So, NFA registered slotted conversion bolts are slightly relieved at the forward upper bolt face to accommodate the normal semi-auto barrel and mounting setup.

With a registered conversion bolt installed, it was deemed acceptable by ATF to allow modification of the lower receiver trigger housing fire control unit any way desired, and most units were simply modified from the semi-auto to selective mode of fire, as described above. It is also perfectly acceptable to swap out the semi-auto unit for a standard factory original SMG lower, as long as the SMG trigger housing is modified (by opening up the mounting pin holes to 9mm), and not the upper receiver center block! Even with a registered slotted conversion bolt installed, upper receiver modifications are generally disapproved of by ATF, although the only specifically illegal modification is removal of the blocking rail on the right upper inside of the receiver. The caveat against modifying the lower receiver push pin mounting hole to bring it back to the 8mm SMG standard is that it may be viewed as evidence of intent to illegally convert the semi-auto upper receiver (by allowing pin-on conversion of an SMG lower housing combined with the first bolt modification method described above) if the NFA-registered bolt is ever separated. It is inadvisable to make this modification in light of that distinction. Guns found with the remaining upper receiver modifications, such as barrel trunnion and feed ramp alterations, are not illegal and add to the perceptible value of the gun if done in a professional manner. An SMG conversion using a NFA-registered slotted conversion bolt is an excellent method of conversion as it does not overly hinder one in obtaining a near copy of the factory SMG original, and in most cases the absolute differences between a bolt gun and registered receiver conversion are merely aesthetic, with the spare parts scenario being the most common complaint. And, if you already have a semi-auto gun in your collection, the acquisition of an NFA-registered conversion bolt may allow an inexpensive changeover into the full-auto realm of the design, as it was originally intended.

The only remaining point to discuss about these registered conversion bolts is as regards their inherent value and weaknesses. All of these bolts were NOT created equal, either in materials or workmanship! If it is decided that a conversion bolt is the way to proceed a careful examination of the intended bolt is in order if you wish to maximize the usage and enjoyment of the converted gun. Slotted conversion bolts were made from two distinct sources, each with its own pluses and minuses. The first and most obvious source was to simply take a factory original SMG open-bolt, and then make the two modifications to adapt it to conversion use in the semi-auto carbine, namely milling the clearance slot, and the bolt face relief cut for the rear barrel support ring. The main concern here is: Did the manufacturer of these bolts properly account for the re-heat treatment of the finished bolt. Since these bolts are normally hardened, it should have been annealed prior to machining, and then heat treated again to proper finish hardness. A finish hardness of at least Rockwell 55-56 on the C (hard, but able to be cut with a file) scale is desired to prevent premature wear of the sear holding notches and bolt face. A tungsten carbide-cutter mill may have been used instead of the heat treatment process (es), but this could lead to localized heat distortion damage (unlikely) during the milling operation(s) if it is not done very carefully. Closely examine any bolts that show evidence of stress risers, hairline cracks, crystallization or burring.

The second method involved new manufacture of slotted bolts, either from machined forgings, bar stock or investment castings. Both forgings and bar stock are common, and perfectly acceptable as base materials for a machine gun bolt, with a slight nod being given to the forged bolts as they duplicate the factory original manufacturing method, while having intrinsically better metallurgical properties. Lastly, some new manufacture bolts were made from cast steel and then finish machined. Investment cast parts are acceptable, providing that the proper manufacturing methods were employed during casting, and were to industry standards. The single most common problem found with cast parts relates to improper cooling practices which allow finished dimensions to stray from spec. All cast UZI bolts were not created equal, and there are unfortunately cast UZI bolts that are known to be off-spec due to a materials specification change which substituted a different alloy than originally specified. This material had a slightly different contraction rate, which caused it to cool to a finish dimension that was larger than tolerance. There ended up being a small lot of these poorly cast slotted SMG bolts NFA registered, and they are quite obvious by the fitting marks that were required to get them to function.

All of these bolts of any manufacturing method, should be heat treated for long-term service, so check the pedigree of the bolt in question and verify manufacturing specs if at all possible. As regards reported, known problems with these registered slotted bolts, it boils down to two areas. First, for the reasons above, some bolts exhibit an excessive wear profile in the sear holding notches which can lead to eventual failure to properly hold on the sear fingers. Potential dangers from run-away firing aside, this is not that serious of a problem to repair, though it entails welding up the sear notches in a jig and then re-machining them to spec (followed by proper re-heat treatment!). Secondly, some of the improperly heat treated bolts have had their fixed firing pin wear down from extended use to the point where there is not enough pin protruding to reliably cause primer ignition. The only acceptable method of repair here is to bore out the old pin and install a replacement fixed pin which can be heat-shrunk into place. These repair pins are rare items, and few people working around MG's have any experience with doing this, but since we are talking about a registered part, repair is the only legal way to go registered slotted bolts should, of and in themselves, be of no particular concern as regards wear and longevity in an SMG conversion, as they have the full potential to meet a normal factory original parts life and utility span, if they were correctly manufactured in the first place. As with all conversion parts though, once the general design was in the public domain many different sources made and registered them and all were not created equal, so a close physical examination is advised. And if a well-worn, or even damaged or improperly manufactured, registered bolt should be found, do not discount it out of hand, as it can be repaired and restored to operational status by a competent professional. The price to be paid should reflect the overall condition, but with registered NFA items getting more valuable with time, any item with a registration paper/tax stamp is worth a close examination prior to purchase, if at all possible.

The final conversion type is the so-called registered receiver gun. As most people understand the meaning of the term, it is qualified by one underlying distinction, the removal of the blocking rail inside the receiver to allow utilization of a standard, solid, SMG open bolt. Though we noted that there are a select few registered receiver guns, as described above, that were converted by other methods, these are not generally perceived to be of the same general class, as they still have the blocking rail intact and therefore have not been converted to SMG description. However, due to their receiver status they most certainly could be completed to full SMG spec at the owner's discretion. The owner who wishes to do so should check with ATF Technology Branch first before doing so - and get it in writing. But for most registered receiver guns, their attraction lies in the fact that they can be fully converted to a near clone of the factory SMG, in both function and appearance. Due to the registration status, with no legal barriers to altering the receiver itself, we are free to return the receiver to its normal SMG description.

This complete conversion of the gun back to its near factory SMG description is accomplished as follows, though individual guns may exhibit various stages of completeness of the full conversion. The complete conversion is begun by removal of the bolt blocking rail in preparation for installation of a factory original SMG open bolt. The IMI design for the mounting of the blocking rail incorporated two extension tabs near each end, that located the rail by mating with opposite slots in the receiver sidewall, and then were welded in place and had the welds ground smooth prior to finishing of the weapon's exterior. When altering the receiver for the conversion to a factory type SMG bolt it became necessary to remove the rail completely, flush with the receiver sidewall, a job best accomplished with a vertical milling cut, as the mill could be controlled to safely remove the rail flush with the receiver sidewall without causing damage. The problem with this is that the last section of the rail was installed too far back in the receiver channel to allow milling it completely free, so some ingenuity was called for. Most conversions were done by carefully milling off the rail as much as possible rearward then using a small torquing force, or shear force, to break free the remaining tab joint. Done carefully, this resulted in no damage to the receiver sidewalls, though many people apparently used alternate methods than a mill, or rushed the job or applied too much torque/ shear force as twisted/dented rear sidewalls are one of the most common flaws in conversions. These sidewall flaws can be straightened, but it is a difficult job and best done by an experienced professional.

The other required major alteration to the semi-auto upper receiver, to render it a clone of the factory produced SMG, was the conversion of the barrel mounting points. The barrel trunnion through-hole needs to be opened up to the SMG factory spec diameter, and properly converted guns will have had this done by line boring to the axis though more commonly this was simply placed into a vertical holding fixture and a drill press was used. This can result in misalignment of the mounted barrel and may be a root cause of feeding troubles, and failure to be able to properly tighten the barrel mounting collar. The rear barrel holding ring/feed ramp assembly will also have to have the entire ring structure milled off flush with the upper surface of the feed ramp, duplicating the SMG ramp contours, to avoid bolt face contact. Once these two operations were performed, an SMG barrel could be simply replaced.

The selector lever block in the top inside shelf of the trigger housing frame was also installed by the same locating tab method, as employed on the blocking rail in the upper receiver. The correct method to remove it was a simple milling operation, though other torquing/shearing operations were commonly utilized. If the block was not fully removed flush, or if any remnants of the weld are left, there may be interference with the elongated right-angle finger of the selector lever that prevents smooth operation of the disconnector function and subsequent improper semi-auto functioning. So, to do a proper conversion, one must completely remove the select lever block, as some improper conversions merely attempted to notch, or cut out, instead of fully removing the block, and a sticking or troublesome selector lever can usually be traced to this. An SMG selector lever is the ideal way to complete a fully correct conversion, and this can be simply exchanged, although the majority of conversions probably used modified semi levers (note: there are two different SMG selectors available - later production ones have the safety tang, early military ones do not.). The semi-auto select levers were modified by adding the third detent, and these detents may give trouble if not properly done. Check for correct depth and spacing as compared to the original two semi-auto only control positions if the selector will not engage the full-auto position smoothly and firmly. A correct job will have the third detent milled, as the factory did, and not hand cut or ground out.

As discussed above the remaining operation to the trigger housing was remarking of the housing exterior for the third selector detent position after it had been milled, cut, or stamped into the housing exterior. The exterior of the trigger housing is where it is easy to tell the care and professionalism, or lack thereof, that went into the conversion. The remarking of the housing for the third detent position can vary from either indistinguishable from factory, to sloppy and badly mismatched. A cold chisel and hand stamp, held by hand and eyeballed, was often the preferred method. The best conversions will have utilized a professional stamping fixture with a font-matching stamp to duplicate the lettering already existing in the other two positions, combined with welding up of the old fire position F, and restamping of the now required R for the semi-auto detent position. Some conversions may simply have had the entire semi-auto unit replaced with a surplus factory SMG type trigger housing, which should be unaltered except for the required modification to the mounting pin holes of the SMG trigger housing. If the original semi-auto sear was exchanged for an SMG sear, with the full width fingers, the sear clearance holes in the bottom of the upper receiver will need to be enlarged to SMG spec for proper clearance. The difference is slight, and many were not properly enlarged as required. For reliable operation with the SMG sear it is necessary to enlarge the holes.

The top cover modifications, required for the SMG open bolt installation as discussed above, will most likely have been accomplished by a simple replacement and exchange of the semi-auto original cover assembly for a surplus SMG, ratcheting type unit. This would be considered the most desirable method for this assembly, though modified semi-auto units are also common and perfectly acceptable, though less military looking.

The final aspect of the conversion is the installation, a drop in job now, of an original solid face, SMG style open bolt and its attendant recoil spring and buffer assembly. This bolt should already come equipped with a wide-lip SMG style extractor claw, but if not it should be exchanged for one.

With a complete exterior refinish, this is the ideal state of conversion in a complete conversion of the semi-auto carbine into as near as possible a clone of the factory produced, select fire, open bolt, SMG original, save for the mounting pm holes for the lower trigger housing and the semi-auto Model A/B markings. These trigger housing pinholes and the receiver markings could have been altered if a registered receiver gun was remanufactured by a Class II manufacturer, but conversions using registered bolts cannot alter the receiver or its factory markings. As can be readily seen, since there exist such a wide range of possible conversions out there, respective values also vary widely. Most desirable would seem to be a fully converted registered receiver gun exhibiting all the alterations to render a near perfect copy of the factory SMG. These guns, circa early 1998 prices, are fairly priced in the mid to high $2000s for excellent condition guns with a known pedigree originally done by reputable sources, to the low $1200 range for a no-pedigree gun with mechanical or major aesthetic problems. Value increases with higher content of SMG style parts, or modifications to accept those standard parts. Registered bolts are fairly priced from $500-$1000 alone, depending on manufacturer and condition, and materials and methods used for manufacture. A complete bolt gun will depend upon the host semi-auto guns inherent value and whether any additional allowable alterations exist. These could command as a package anywhere from the low $1000s to the mid $2000s. As a general rule ANY bolt gun is worth less than a registered receiver conversion of comparable condition. Though, if you happen to run across one of the few very early conversions, those that fall under the heading of anomalies, such as those that contain a NFA registered selector lever, or are grandfathered un-registered slotted bolt, judgment will have to be used as to value based upon overall condition and the wear and tear on the registered part(s). Negotiations are in order, as there is simply no definitive value basis.

Hopefully now, with an understanding of the myriad definitions extant of the Conversion UZI, a clear picture can be drawn of the complex nature of this particular class of civilian legal NFA firearm and its convoluted path to the marketplace and firing lines across the country. While there exists a large variance in this class of guns, it is also nice to know that they represent a solid value for the collectors money, while being easy to shoot and service, as spare parts and accessories are abundant and inexpensive.

Originally published in the May, 1998 issue of Small Arms Review magazine.

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