Saturday, May 7, 2011

Some work on the Suomi M31

Well, I haven't been able to blog much lately due to the school semester winding down. I've been real busy getting ready for finals, and had a lot of other stuff to tend to. I managed to get either a really nasty stomach bug or a case of food poisoning that lasted for the entire weekend of Easter. A little time in the hospital on an IV got me back on my feet. I took the following week off of working on anything, despite how badly I wanted to be in the garage tinkering.

So, last week I worked on the firing group for the Suomi M31. These are an open bolt design from the factory, and due to ATF regulations, I cannot return them to open bolt operation. For those who aren't in the know, the main two designs used by firearms manufactures are open bolt and closed bolt. The main difference between the two is exactly what it sounds like; the closed bolt fires the cartridge with the bolt closed on the round by means of a hammer or striker system, while the open bolt system has a fixed firing pin on the bolt itself. An open bolt system is very easy to design and build, as there are fewer working parts than the closed bolt system. A sear simply holds the bolt back until the trigger is pulled, releasing the bolt. The bolt slams forward, pushing a round out of the magazine and as the bolt closes on the round, the fixed firing pin dents the primer when the round is fully seated in the barrel. It is very similar to a striker system, but a striker system has the bolt closed on a loaded round and pulling the trigger drops the sear, releasing the striker pin, allowing it to slam forward on the loaded round. A striker system is legal to use on one of these rifles, but I already had a AR15 fire control group laying around, so I used it. This modification requires a lot of work to the rifle, but it cannot easily be converted back into a machine gun once completed. Stupid Hughes amendment. If it weren't for that unConstitutional piece of, I would just pay the 200 dollar tax and reassemble this rifle into a select fire machine gun. However, I don't want to be a felon.

Anyway, open bolt is out. In order to make this a closed bolt rifle, I had to mill out the bolt for a hammer to clear. This is very difficult to do on these rifles, as the bolt has been hardened to an unbelievable level. I have heard that these are in the range of 50-55 Rockwell, which is VERY hard. I can't even put a scratch on the bolt with a file. The only thing that will cut through these things is a carbide bit on the lathe, or a TIAN coated carbide end mill in the milling machine. I had to do both, since it could not be set up for easy conversion back to the original open bolt setup, I had to make the receiver a bit smaller diameter than factory, and machine the bolt down to match. The factory bolt is ~1.2" outer diameter, and the receiver I made was 1.125" inner diameter. This required very light cuts of less than .005" to keep the heat down and keep from dulling up my carbide cutting bit.
Another problem with these bolts being so hard is that although the fixed firing pins in these are removable, the inner diameter of the end of the bolt is very small. Too small, in fact, for an AR firing pin. Well, as luck would have it, I tried to press out the fixed firing pin but only managed to get my punch stuck. In trying to remove the punch, I screwed up the bolt. :( So, machine another bolt I had laying there down, mill out a slot for the AR hammer, and try to get the firing pin out of this second bolt. I finally got pissed off enough I decided not to press the fixed firing pin out, I just grabbed a 1/4" TIAN carbide end mill and chucked it up in the drill chuck and bored out the whole thing. I then machined a piece of M2 drill steel down to 1/4" OD, drilled it out to 1/8" inner diameter, and pressed it into the end of the bolt. After this, an AR pin fit real well in the bolt.
Here's the end mill boring out the end of the bolt.

EDIT: pics added of the machined bolt

The extractor end of the bolt with the new drilled and turned piece in place.

Then, I had to clamp the bolt down in the mill and mill out a slot roughly 1/2" wide and 2" long for the AR hammer to cycle and clear the bolt. I had already done this and turned the bolt down previously, so all that was left was to fit the AR hammer, trigger, disconnector, and trigger linkage into the Suomi trigger housing. One problem- the AR hammer is wider than the Suomi trigger group by almost 1/2". Since the hammer springs have to fit on the hammer axles, I couldn't narrow the hammer any. This means I would have to cut some steel plates out to weld on the Suomi trigger housing that would widen the whole thing and allow for the AR group. I had to cut some side plates that would weld to the receiver and allow the trigger group to be bolted to the receiver.

A pic of the modified trigger group in place with the receiver side plates welded in and ground down behind the trigger group.

The widened trigger group with AR components in place.  It's a little rough still, but some sandpaper will fix that up.
Notice that the Suomi trigger is not attached to the hammer assembly yet. I have to build a rocker arm of sorts that will allow the Suomi trigger to activate the AR hammer assembly. I had to trim the AR trigger off of the sear piece to get it to fit.

There's a bit more to this, but apparently I haven't put the pictures on my computer yet. With the receiver pieces welded on, and the trigger group done except for the rocker arm assembly, the rifle now bolts together, the bolt slides back and forth in the receiver, and it strips rounds from a loaded mag or drum well. Over the next couple of weeks I hope to mount the rear sight, finish the trigger rocker arm assembly, and maybe machine an ejector, harden it, and get it installed. Thankfully, finals are this week, and once I'm done with those, I have several weeks of rest and tinkering ahead.

I'm at a standstill on the .50 BMG. I am still trying to arrange a decent buttstock for it, and come up with a better idea for the scope mounts. I may well build a new receiver for it, because I have another 3' of the tubing needed for the job. I made the AR hammer and trigger assembly for use with the current receiver, and found out that where I mounted the assembly was too far forward. I've done so much cutting and welding on the receiver that there are high and low spots inside the receiver that makes the bolt stick a little while cycling. I don't like this, and I don't have anything long enough to reach deep inside the receiver and knock those spots down. Since I am not happy with a few things, I am thinking of just chalking up this receiver as a lesson learned and try again. I'm out 20 bucks to learn the lesson this time around. It shouldn't take a couple hours to machine the slots in the new receiver, since I still have the CAD file to print out and glue on the receiver for a guide.
I probably should have mapped this out a little better, but I'm enjoying the build still, and I don't mind breaking a couple eggs in order to have a final working design that I'm happy with in the end.

Anyway, I'll get some more stuff built and posted over the coming week.


  1. Sweet! I've been thinking about a way to convert a suomi from open to closed bolt fire, and i think I'm going to do something like this, but with some parts from maybe an Sks? I was also wondering if it would be possible to place the hammer at the back of the bolt, to avoid me smashing my head off from a wall while trying to mill the hammer slot in the bolt.

  2. Brando,

    Thanks for the kind words.
    I've responded to your questions in the other thread you commented in.
    Good luck building your M31, if I can help, just let me know!