Sunday, July 22, 2012

A finished, Finnish Suomi M31. From scrap steel to shooting steel.

I've meant to put a few pics of a completed Suomi M31 rifle for a while, and I'm just now getting around to it.
I machined the diameter of the bolt, machined a new firing pin, bored out the fixed firing pin, machined off the bolt feed lips, stretched the barrel, bored the barrel shroud for the barrel, cut the rear receiver threads, and rebuilt the trunnions for this particular rifle, the owner did all the welding and completion machine work on it.
The stock isn't the nicest, but it serves the purpose.
This rifle meets 922R compliance, and out of the following

27 C.F.R. 478.89 lists 20 parts:

(1) Frames, receivers, receiver castings, forgings or stampings (USA)
(2) Barrels (import)
(3) Barrel extensions (USA)
(4) Mounting blocks (trunions) (import)
(5) Muzzle attachments (NA)
(6) Bolts (import)
(7) Bolt carriers (NA)
(8) Operating rods (NA)
(9) Gas pistons (NA)
(10) Trigger housings (import)
(11) Triggers (USA)
(12) Hammers (USA)
(13) Sears (NA)
(14) Disconnectors (USA)
(15) Butt stocks (import)
(16) Pistol grips (NA)
(17) Forearms, hand guards (NA)
(18) Magazine bodies (import)
(19) Followers (import)
(20) Floorplates (import)

We have 8 imported parts, which is less than the maximum of 10.
None of the original full auto parts work in open bolt/full auto operation anymore, nor will full auto parts fit on this rifle. The trigger housing has been permanently modified to prevent conversion to open bolt and now uses a hammer, the receiver diameter is reduced so that an original bolt will not fit, the bolt is slotted to prevent use with a full auto sear, the feed lips on the bolt are machined off, the fixed firing pin is removed and a floating firing pin installed, the receiver nose has a smaller diameter so a short barrel will not fit in the receiver, and the barrel is stretched.

After I did the work that I did, the owner came to my shop and used my tools to finish building the receiver. Once he had done the rest of the welding and machine work, we cleaned up the welds and machine marks, then put all the pieces together. The owner is planning to throw some Duracoat on it and give it a factory-fresh look.
Some sanding marks remain in the area where the two receiver sections were welded together, but the owner says he can get those polished out on his own.

Not bad, since this is what we started with.

Here, the front trunnion, the ejection port, and the receiver nose have been completely rebuilt, while the rear mag trunnion has been rebuilt, but not fully shaped or ground yet.

Here are two new receiver sections, one in place on a rebuilt receiver section, the other being used to support the drum. I machine these a bit long, and allow the end user to trim them to size before welding.

The barrel extension now protrudes from the barrel shroud.

Cleaning up and polishing the welded, ground, and machined areas.

 All together now, ready to be welded into a single piece, then have all the remaining machine work done.

Mocking it up in a stock to make sure the receiver is the correct length.

Here is the rear receiver, with a template glued on to it. The receiver can be welded before being finished machined, or the other way around. As long as everything is aligned beforehand, it is unimportant which you do first.

The same template/receiver, but with the glue releasing the edges a bit. The centerpunch marks all align, and that is what counts.

And here's the rebuilt front trunion with the barrel shroud attached.

Finally, here's the complete rifle.

I kept the removable barrel shroud feature. Turn that lever in front of the drum and the barrel shroud can be twisted and unlocked, allowing it to be removed. The Finns did this so they could swap out barrels with ease.

That's from a 70 dollar parts kit.
There's nothing like doing a mag dump when you have a 72 round drum of 9mm available.


  1. Dude, you do awesome work!!

    1. Thanks Six!
      I can only take so much credit for the work shown here, since the owner of the rifle did some of the work as well.

  2. Oh. Looks great!
    Is it ready to go bang? :)

    1. Thanks Hyperprapor!
      Yes, it goes BANG!
      Very accurate, no recoil. Lots of fun :)

  3. Okay, you have got to know my question.......... you bringing it to DABII?

    1. Belle,

      I probably will bring one like this, but not this specific one. This rifle is owned by a friend of mine, I helped him built it from scrap metal into a functional rifle.

  4. Beautiful work!
    Is that the original stock? The pattern/grain of the wood is gorgeous.

    1. drjim,

      Thanks for the kind words! I won't take full credit, the owner of the rifle did the remaining 20-30% of work after I had done my part, but I'll be sure to pass along everyone's compliments to him as well.
      That is one of the original stocks that I have laying around here. I use it for test fitting more than anything. Believe it or not, this is not one of the best looking stocks I've seen for these old subguns.

    2. Almost looks like a burl. I don't think I've ever seen a stock grained like that.
      But then there's an awful lot of things I haven't seen in the gun world.

    3. You know, I haven't seen many like it, but I'm no expert, either.
      If we were talking about different kinds of steel, I could do some analysis on them to tell you more about them, but I'm pretty lost when it comes to wooden gun stocks.
      I have four different kinds of stocks for the M31, all sitting in my shop. None of them look the same, but all but this one are "normal" wood grain.
      A friend of mine who is a bit more knowledgeable about these things says it might be some kind of burlwood.

    4. That looks like some kind of figured maple to me. If not "pre-popped" the grain will give you those subtle transition dark lows and light highs. It gets extremely interesting if you give it a very light oil/varnish blend base coat before you stain it :-)

    5. I believe you are the exact person I was going to ask next.
      Perhaps I should bring this stock over to you and let you work some BaconFatLabs magic on it?

    6. I'd be more than happy to! The garage is getting a good bit of room in it very soon so I can start taking on fun projects again.

    7. I know a guy who needs you to look at his Hobbycnc boards.....

  5. I pray you are just being rhetorically sarcastic.

    1. I'll just give him your contact info and let you decide how sarcastic I'm being.

    2. Okay, okay. As long as it doesn't require a drive to Decatur :-)

  6. An absolute work of art. And that describes both your restoration, as well as the work of the original stock maker. The result is astonishing and beautiful.


    1. Good to hear from you again, Farmkid!
      Haven't heard from you in a while.
      Thank you for your kind words.

  7. Are you still making the m31 receiver and if so how much to buy one? thanks

    1. trigun,

      I never made a M31 receiver for sale. I used to machine all the parts for the M31 other than the receiver and as shown in the photos I would rebuild the front half and machine a new rear half, but no longer do that for anyone except close friends.
      To my knowledge there are no 100% suomi receivers on the market. Hellbox armory makes a great 80% receiver and only requires some dremel and file work to finish into a complete receiver.

  8. I have a CUT kit. Do you sell the necessary parts to get it going at full auto?

    1. 1. These were ALL originally full auto.
      2. ALL of these were CUT kits.
      3. Read up on the laws before you ask me to commit a felony.
      4. Not just no. Hell no.