A Domestic Yugoslavian Subgun with German Heritage
As it's model designation implies, the Yugoslavian M56 was designed in 1955-56, going on to serve with the JNA en masse starting in 1960.
It's simple tube construction and overall streamlined manufacturing process set the M56 up to replace the M49 series of submachine guns which eventually took place in 1975. Still chambered in the Soviet Bloc sweetheart 7.62x25 of it's predecessors, it proved to be a rather reliable design and can still be seen in conflicts across the globe from time to time. These parts kits have been on the US military surplus market in waves over the past 15 years in varying degrees of completeness; some with barrels, some without, some with all the receiver pieces, some with none. As with most subgun kits, they're still relatively inexpensive but also lack the definitive support for build and conversion parts as compared to some of the more prolific designs on the market.
This build proved to be a long process that went through several redesigns, but ended up with a tidy self contained semi auto conversion that works well with a rewelded tube and denial bars. Video embedded at the end of this article.
Step One : Tube and Barrel
It's very obvious when looking at the M56 where the German influence went and from afar most people would even mistakenly identify it as an MP40. The plastic encased lower receiver combined with the wobbly underfolding stock and magazine placement can make it a little tough to distinguish, however the length of the receiver tube is the biggest giveaway even to the uninitiated. There's a lot of room to locate your bolt denial bars, I chose a 1/4" piece of stock at the 12 o'clock position at the rear of the tube. The disconnector opening will need to restricted by closing off about 1/2" with a welded plate. I chose to use a piece of flat bar stock for this that also worked as a denial key for the lower. At this point, this receiver will only accept a permanently modified semi auto bolt and lower while only fitting onto a semi auto lower. A turned slug of aluminum is the best thing to use as a backer for the welding process, with a step turned to line up the front trunnion square with the rest of the tube. The three holes on the bottom of the tube interface with three corresponding screws in the lower and their location on the lower can help line the other sections up for an accurate overall length. The ejector can be a little tough to remove for inserting the weld backer, the pins holding it in place are staked. A few hard hits with an undersized punch should get them to move enough to swing the ejector down and out of the way.
Installing the barrel is easily done by lining up the center of the breech face with the ejector and then drilling the locating spot for the set screw. The front sight uses a taper pin - if it won't move off of the old stub from one side, try the other. Once the barrel nut is torqued down, don't forget to peen over the flat washer to prevent the nut from backing off. I welded on a barrel extension to make this a carbine build; 16" barrel and 26" or more OAL with the stock fully extended. In the future I might P/W a Form 1 direct thread suppressor on to it. As a result, I omitted the bayonet lug.
Step Two : Lower and FCG
The lower receiver assembly is a pretty straightforward design and I really like how it was designed with so few parts. There's only two springs driving the entire mechanism (three if you count the selector 's flat spring) and 6 parts total. When taking off the plastic side covers, be careful not to drop them; the thin edges that ride along the tube are very prone to breaking.
Converting the FCG itself to semi auto only is very easy once you understand how all of the pieces interact. The trigger bar is also the disconnector in this design and it accomplishes that task using a moveable contact point. In semi auto, the contact point it held by the selector at a 90 degree angle upright in the tube directly in the path of the bolt. Once the trigger is pulled the sear drops, bolt goes forward, completes it's fire cycle, returns, and trips the contact point forcing the sear back up into the path of the bolt catching it before it can go forward again. In full auto, the selector simply moves this contact point 45 degrees out of the path of the bolt. Welding the contact point in the 90 degree position creates a permanent disconnector on the trigger bar, and welding the selector in place ensures that even an unwelded trigger bar cannot function in FA. These steps combined with restricting the disconnector slot and keying the lower ensure that this build is permanently semi auto only.
Step Three : Bolt
Unlike it's Germanic cousin, the M56 uses a coil mainspring and a fixed firing pin turned into the bolt face. This helps make the conversion process a little bit easier since the empty space in the bolt can be used to house the new striker system. Initially, I had floated the idea of converting the FCG to hammer fired and using a linkage to make up for the awkwardly long distance in the lower, but the end of the bolt was still too far forward to make it work out quite right. Combine that with a full diameter mainspring and I decided to settle on the striker mechanism.
After milling off the FA sear catch and milling the top to accommodate the denial bar, the bolt was through drilled using carbide tooling. Since the firing pin is a permanent feature, the entire bolt was hardened and can be a little too tough to machine using HSS and/or cobalt tooling. A slot was milled into the center of the cavity in the bolt, this would be the opening for the new sear catch. A piece of aluminum was turned to match the bolt's ID and then slotted for a piece of key stock to be pinned into place. The new firing pin was a piece of O-1 drill rod, profiled, hardened, and then pinned into place. A piece of aluminum was turned and slotted to allow the new striker to move within the bolt body, and then drilled for a roll pin to act as a cap hold the striker and it's guts into the bolt.
Step Four : Testing and Troubleshooting
One of the biggest failures of the initial design was using the tail of the firing pin as an anchor point for the striker's spring. The springs kept getting bound up and destroyed after only a few rounds. I found out that as the springs were settling into place, they would compress just enough to sit crooked on the firing pin tail and then get crushed on the next cycle. I ended up using the cap as the new anchor point and swapped over to a larger diameter spring; the binding problem was solved. After a short testing period a new problem emerged, the key stock I chose for the striker was too flexible and was slipping off the sear during recoil. Welding a piece of flat O-1 to the spine of the striker stiffened it up and stopped the sear slippage. Then testing showed a much more serious problem; out of battery discharges. Using the HFR setting on my camera, it turned out that the bolt was bouncing when it would run home and leave it just barely out of battery but enough to get bulged cases. The recoil spring I was using was trimmed 6 coils initially to make up for the now "shorter" bolt, but that was too much. A new recoil spring from a different kit solved that problem and didn't have to be trimmed at all - it pays to try everything as it is before trimming when balancing springs and my own impatience had me skip that step.
The last and most pervasive issue that still somewhat persists in this build - feed stoppages. I standardize all of my subgun builds in 9mm Para and for the most part have very few issues as far as FTF and FTE on the rechambers of native 7.62x25 designs. The case heads are nearly identical and most feed stoppages can be traced back to bad magazines, as is most noticeable with PPSh-41's and M49/57's. However, the 9mm barrel currently available for the M56 has bad feed ramp geometry - after continually cycling through different batches of magazines, the stoppages only lessened when the feed ramps were deepened and widened with the help of a dremel and black emery compound. The combination of longer magazines and the original feed ramp design seemed to be the culprit. They still happen to an extent, causing the occasional FTF which is acceptable for me but maybe not for some.
After all of these cumulative problems were ironed out, the M56 turned into a very fun carbine that handles quite well for looking so ungainly. I've taken it to the 50 yard line and rang the gongs without much trouble, but still have yet to test out the 200m sight leaf.
Conclusion
There is a saying that states "The cheaper the parts kit, the harder the build" and in most cases that's right, this build was no exception. As seen with other builds like the PM-63 Hammer Fired Conversion, there are plenty of different ways to think outside of the box and overcome the lack of a dedicated conversion market for these cheaper kits. This project did give it's fair share of headaches but has turned into an enjoyable build with some very cool historical provenance. The overall lines and style are a far cry from the svelte wood stocked M49/57, but the M56 has a certain industrial feel that is hard to beat. Next time you're looking at war footage from the last 30 years, see if you can spot "The MP40 at Home" on the frontlines - they're hard to miss!
