Yugoslavian M-56 SMG semiautomatic conversion                                                                  home

Overview:

     The purpose of this site is to help develop the best method of converting the M-56 smg into a legal semiautomatic from a parts kit.  It is an "in progress" step by step guide, intended to help people (much like myself) who have little to no knowledge of part kit conversions, to feel comfortable through the building process. As of this time, most of the information will be logged here, randomly, and when things get sorted out and 100% decisions are made, I'll try and break them down into different categories so that you can follow along in your own order...or for the most part anyway.

Some Ideas on Building the Yugoslavian M56 as a Semi Automatic

Being new to building semi auto long guns, it seemed challenging to build an AK so I set up to buy some kits and learn.  During my looking, this very interesting Yugoslavian Sub gun appeared and not having a clue, I bit.  When it came, I looked at the parts and did a clean up being very pleased with the condition of the weapon and the bore but as far as building it as a semi, even reading some of the posts convinced me it was beyond the typical AK.  So, into storage it went.  But, realizing I had also invested in an 80% receiver and the bolt work, I decided to look at it again.  I knew nothing of tube guns but this is what I have been able to glean from studying the firearm and reading various posts.  This compiles a few of my experiences and ideas but incorporates a lot of ideas from various members of the Weaponeer.net group buy forum all of whom are mentioned below.  This is not offered as a complete solution but as a suggested approach of converting this interesting little weapon from a full automatic to a legal semiautomatic weapon.  It does not address the compliance parts issue at this time but only a proposed method to convert to a semi automatic.

A.  General observations.

1.                  It is a very simple tube type receiver weapon.  The bolt and firing pin are integral.  The trigger mechanism drops a small metal “hammer stop” or “stop hammer” which locks the bolt.  When dropped, the bolt is released and is pushed forward by the recoil spring. The springs kinetic energy and the bolt weight strips a round out of the magazine, slams the bolt into the chamber at which point the firing pin hits the primer, and the blow back from the explosion of the cartridge drives the empty cartridge back to a hook like ejector at the base of the tube, ejecting the empty cartridge through the top of the tube receiver and then it cycles again until the trigger is release, at which point the stop comes up and grabs the bolt.  The weapon will fire FA as long as there are bullets and the trigger is held back.  The key to making the weapon semi automatic are:

a.      Devising a method to run a floating fire pin, or spring loaded firing pin through the bolt as it appears the existing bolt pin combo is only suitable for FA fire and thus technically illegal and also would be open bolt which is illegal.

b.      Utilizing the existing trigger linkage to utilize a hammer or striker FCG;

c.      Determining where the bolt is throughout the firing cycle for purposes of locating the FCG.  This is because the striker/hammer will have to work through a notch machined in the base of the bolt and it appears that location will be critical in cocking the striker/hammer and releasing it to strike the firing pin.  The reason this is critical is the weapon can only fire when the trigger is pulled making it fully automatic.

2.                  My next task was to take the thing completely apart for two reasons, when you see how something is put together, it is easier to modify or see if it can be modified.  Secondly, Yugoslavia must love Cosmoline.  It is the only way to get it clean.  Then I reviewed everything available on converting these to semi automatic tube guns so they work.  I will discuss my experience to date and what I have discovered by reading other sources regarding ideas on making the semi conversion.  Any other observations are welcome.

B. Disassembly.

3.       Stock/lower receiver. 

a.       The stock unit has a plastic cover over a wooden block in the front and a stamped steel frame making up the back.  There are also plastic pistol grips.  The plastic cover and the pistol grips are each held in place by two screws with those god awful spanner nuts on the back which holds the plastic assembly on the upper and two screws with those god awful spanner nuts on the pistol grip. I used a screwdriver filed so that it had two prongs to fit the spanner nuts. Carefully clamp the unit in a vise making sure not to put too much pressure to avoid cracking the plastic; just enough to hold the unit firm while you remove the screws.  Otherwise you will need three hands.  Make sure you make the prongs long enough and make certain you get a screwdriver with a blade that will fit the hole in which the nut is countersunk.  Too narrow a blade makes it difficult. One screw goes through a wooden block which on the forepart of the unit, the second screw passes through an aluminum spacer on top of the trigger assembly. The two in the pistol grip simply attach it to the metal frame. As you remove the screws clean them and screw the spanner nut on the screw after you clean the cosmoline and crud from the threads.

b.       The next step is to remove the folding stock which must be done before you can remove the trigger assembly.  The button on the left side unscrews.  Be careful as you remove it as there is a strong spring just below the screw and you don’t want it launching off into space and losing it.  You can then remove the folding stock.  Set it aside.

c.       The trigger assembly can now be removed.  First remove the spacer and put it in the bag.  You will note at the front where the release hammer/stop is located there is a spring.  This spring is threaded through a pin at the lower part of the unit.  There is also a large pin which holds the hammer/stop unit.  Driving out both of the pins will release the front part of the trigger unit.  You can then move to the back of the unit and remove the pin which was covered by the stock.  This pin has a flange and can only be driven out from the left hand side on my unit.  Check it carefully before removing the pin.  Remove the pin directly ahead and slightly above and the pin which holds the trigger in place.  You can then move the trigger assembly forward and out of the notch into which it is fitted in the flat piece of metal with the strange writing on it.  That is held by a small spring on the back with notches on the top but they all come out rather easily.  The trigger assembly will also drop out at this point. 

d.       The entire stock unit is now disassembled except for the wood forepiece.  I could see no reason to remove this unit.  Its primary purpose seems to be to hold the tube receiver by way of the three screws on the top.  Just clean it well.

          4.  Barrel/magazine well/trunnion. 

a. Barrel.  The barrel should still be attached to the torched receiver and the cut should go just behind the magazine well.  You can actually put your magazine in the well and if so, you should have no problems when you reassemble the weapon, the cutter didn’t mess up the magazine unit. You will notice a set screw just in front of the remains of the receiver where the barrel comes in on the top and a little notch on the bottom.  Credit goes to Tom, aka Tommerr on the Weaponeer board for the following and I will simply quote him on removing the barrel:” The set screw has a pin on the bottom which locks into a hole in the barrel. The square holes are machined notches into which the lock washer, which sits between the trunnion and the barrel lock nut, is keyed. Under magnification, you can see how the washed is punched into the slot. Use fine, pointed tools to pry the washer out of the slots.  

“. . .I was able to unscrew the barrel from the trunnion. The trunnion was still welded into the torch cut receiver. The barrel screws into the trunnion from the INSIDE of the receiver!! Between the trunnion and the barrel lock nut is a thin washer. A small cut is machined into the trunnion and into the barrel lock nut. After the lock nut is torqued, the thin washer is punched into the trunnion cut and into the lock nut cut. This turns the washer into a lock washer and it WORKS!!

“The barrel nut is 24mm. It is tight! Utilize the bayonet lug when clamping the barrel between blocks of wood. I did not use a 24mm wrench to remove the barrel nut. I used a tight adjustable wrench. A perfect wrench fit is the key to minimizing nut damage. I may alter a 24mm, six point sockets to make a clamp fixture. A partially split socket, tightened with a bolt, may do the trick.”  

“When you remove the bayonet lug and front sight, I recommend dremeling the pins on both sides. The pins were tightly set and also bent in the process. Removing both ends of the pins helps reduce press out loads. You may have to drill the center of the pins to reduce stress.”

A couple of observations on the barrel, it is too short, you will have to either extend it or you will have to turn a new barrel.  Or you pay the tax or build this thing as a pistol to stay legal.  I lean towards a new barrel or an extension.

 

6. Magazine.

This was easy, simply remove the floor plate and spring, note the position of the follower when it comes out and clean the entire unit of cosmoline, oil and reassemble.  The bullets should fit tightly into the magazine.  It is double stack.  If they don’t fit tightly there is a good chance you have the follower in backwards.

 

 

C. the Receiver and Bolt: 

 

          1. As stated above, Prexis/Sten is making an 80% receiver for this unit with a reduced ID.  That means you will have to have the bolt machined to fit the reduced diameter receiver. For an additional cost, he can machine your original bolt to match the reduced ID...which reminds me that I need to send mine out to him.

 

2.              The Bolt.  This is the key piece to making the weapon semi auto.  WEIGH YOUR BOLT.  YOU WILL BE MAKING MODIFICATIONS TO THIS BOLT WHICH POTENTIALLY WILL AFFECT THE WEIGHT AND YOU WANT TO MAINTAIN THE ORIGINAL WEIGHT AS MUCH AS POSSIBLE.  THE WEIGHT OF THE BOLT AND THE RECOIL SPRING WILL DIRECTLY AFFECT THE OPERATION OF THE WEAPON WHEN IT CYCLES.  The bolt is hollow for about ¾ of its length. The recoil spring is fed into this hole and there is a flat machined spring holder that fits inside the bolt cavity.  Place this in your small parts bag. The front of the bolt is machined with several rails and moves approximately 2 inches when it cycles.  The Charging handle is located on the right side and there is a hole drilled through the bolt into which the charging handle is inserted.  It is made so the handle can swivel within the confines of a machine notch on the right side of the bolt.  There is a sheet metal hook which attaches to a little flange on the receiver when it is swiveled forward (it should be there as it is just in front of the cut), which presumably acts as a safety of sorts. This means the bolt must be able to swivel and this is accomplished by the square extension being able to swivel in the bolt hole.  That square extension must be replaced because it interferes with replacing the existing firing pin (FP) with one drilled through the bolt.  It is obvious the existing FP machined on the bolt face will need to be removed. Then a new FP hole needs to be drilled through the bolt.  After study, it seemed the best thing would be to machine a plug that would fill the hole and attach the handle so that it could swivel.  This could be done by shortening the existing handle and fastening it to the plug in a fashion to allow it to swivel.  Possibly by machining a grove in the shortened extension and pinning it to allow it to swivel within the existing notch.  Below is a rough and crude drawing of how this would work.

The FP channel then can be drilled through the plug and the charging handle, retained by a pin, can still swivel within the notch already machined into the bolt. 

2.      Full Auto to Semi.  Once the issue of the handle extension is resolved (and as of this writing it has not been tried yet, the following methods can be used for making the weapon a legal semi.   There is a concept design on the Prexis web site http://www.prexis.com/sten/semistendesign.htm which potentially might work.  It is produced here with the permission of Prexis.          

NOTE: This design has not been reviewed by the BATF and is offered as a "Concept" on a way to approach a semi-auto build. Remember, any build must meet NFA/BATF guidelines.

All ATF/NFA rules apply to purchases made from this site.  The information/products in these web pages are not intended to be used to construct illegal firearms. For legal semi auto and dummy gun information, contact BATF at: www.atf.treas.gov

In addition, a Weaponeer forum member, EZ Feed, proposed a similar conversion.  To quote: :

”Now here is your ticket to a 100% legal semi auto that you do not have to worry in any part about having reclassified.

Info below and final copy pending Copyright 2005 Weaponeer

My best guess on this would be to find a new undersize I.D. piece of DOM tubing for the build. 1/16th under would work nicely. Next turn the bolt to fit the new I.D. and drill through for your floating firing pin. Be sure to construct a skirt much like the MG 42 semi bolt has but be mindful of bolt weight and length.

Once you have constructed a skirt long enough to handle the arc of a modified AR hammer then weigh it and subtract that much off the tail end of the bolt by either grinding or cutting. A little more I don't think would hurt but be sure that you don't lighten the bolt too much or else you will lose the momentum and weight needed to strip the cartridge from the mag on the next round. If you lose too much weight the bolt will slow down too much while it pushes the cartridge from the mag and this will cause feeding problems.

If you don't want to make a skirt then the next best option would be to take a 3/8" end mill and plunge into the rear of the bolt as shown in the illustration below. This will undoubtedly be a bit easier but may affect feeding from excessive reduction of bolt weight. You may have to add a bit of lead to bring it up or near specs again. WEIGH YOUR BOLT PRIOR TO MODS.

Info below and final copy pending Copyright 2005 Weaponeer

for illustrations click links [copied] below. Sorry bout the crudeness guys!!!!!!!!!!!!!

  

 

  

CONCLUSION.  This is as far as I have gotten and my bolt goes off next week for modification so it fits the new reduced I.D. receiver. In the meantime, I intend to work on the trigger and fire control so I can see where I have made mistakes or wrong assumptions or, if it works so I am ready for the bolt alterations upon its return.  The FCG utilizing the existing linkage is a key to making the entire semi auto work.  The bolt modifications can then be made once that sets in the stock.  It should then be a matter of machining the plug, the modifications to the charging handle, the firing pin channel and any retaining pins that may be necessary, together with the skirt on the rear of the bolt, if necessary and modifications to the spring, if necessary.  It is a challenging build but it should be worth it.  I haven’t got to parts count yet either for compliance purposes but will work on that while I wait for my 80% receiver and bolt. 

If anything else comes up, or if any of this does not appear to work or be feasible, feel free to send me an email.