Suomi M31 SMG

I had initially planned to convert a Scorpion Evo as my host for torture testing prototype suppressors, but those don't come cheap even if I were to make the conversion parts myself. Then I considered doing a 9mm AR, but plenty of those have issues, especially converted post samples, and it's also pretty boring. Then it dawned on me; I had bought a Suomi parts kit with barrel & vertical cut receiver from a guy a few years ago for $100, including barrel, two stick mags and one drum mag. I was more interested in my scratch builds at the time, and the Suomi is pretty heavy and awkward to be much fun as a range toy in a world of 4-7 pound, ergonomic, optics-ready carbines. But as a host for prototype destructive testing, a vintage SMG that I'm into for peanuts made a lot of sense. So I got busy cleaning up torch cuts, mocking the parts, figuring out fixtures.

Unfortunately, I'm not good about documenting my endeavors, tend to get hyperfocused on the task at hand and don't think to take any photos, so I don't have any in-process pics, but if you look at any vertical torch cut Suomi kit, you'll know exactly how this one was received. They're a mess, with one cut through the center, another through the rear cap threads and the 3rd right through the barrel trunnion, all of them 3/16" to 3/8" wide, sometimes flaring even more at the ends. The cuts through the rear threads on this one took out a good 20% of the material. Luckily, the one through the front trunnion was kind of far back as these go, didn't get into the lugs for the combination hand guard, barrel retainer.

I spent probably a good 7 or 8 hours setting it up, welding and re-machining. The welds certainly aren't invisible, but I was able to blend them pretty well. And thank the lord for good eyesight, as even with the horrid torch cut through the rear, I managed to line up the threads well enough that it was just a matter of thread filing the weld after clean up and the cap went on

The ejection port did end up enlarged slightly, as the difficult-to-hold-in-a-vise-at-an-angle receiver climbed up the end mill out of the vise jaws and got some gnarly gouges in the port. At any rate, this is what I ended up with:


Of course, one of the issues with using the Suomi for this purpose is that the barrel is basically flush with the end of the hand guard. So I threaded the muzzle 5/8-24 and then made a short adapter with the same threads which stick out just far enough.


And Viola! A 13 pound conglomeration that can get a pistol can nearly as hot as a rifle can. So far my cans are holding up to 143 rounds at ~ 700 RPM (two sticks and one drum). The real abuse will begin soon, as those other 4 sticks and 2 drums just came, which will allow me to fire over 400 rounds in just a couple of minutes. I figure that's more than sufficient to call them full auto or even belt fed rated!



As an aside, the considerable heft of this thing combined with a fairly moderate cyclic rate makes it a very controllable package. I just got it done, so haven't had a chance to do any real range work to comment on accuracy, but it does run 100%, not a single failure with the sticks or the drums. It does not have a semi-auto function, so accuracy testing will be interesting! But I have other platforms for testing POI shift or changes in precision with the cans; this one is just for the durability component.

A couple pics of the welded receiver:

 

Always wanted a Suomi, even though its heavy and clunky, I just love the drum mags in them.

 

Yeah, the drum mags are well designed and well made. They're also about the cheapest drum out there aside from plastic rimfire stuff. I paid $60/ea for those 71 round drums. Basically, everything you see in the photo-the SMG, 6 stick mags and 3 drum mags-I'm into for $320 if I don't assign a value to the full day I spent building that receiver.

If you decide to take one of these on, I'd suggest going for one of the new "80%" receivers. Even if you get a vertical cut one, it's a lot of work, not just simple welding with a little dremeling. The torches make a mess of these things, and all the slag that spills into the inside of the tube and fire control area is not easy to deal with. I attacked the trunnion first, as that's the most critical part. After carefully welding to effectively fill the huge gap through a very thick piece of steel, it went in the 4 jaw on my big lathe to clean up the bore, which is stepped. Then I had to mill the ejection port back out. The ejector had been touched by the torch, but was still in-tact enough to use. The center and rear cuts were equally difficult with the thinner material, took many passes with cool down periods, and despite one's best efforts, that much heat and filler is gonna tweak things. I spent a good while with 240 grit wrapped around a mandrel before the bolt would slide freely. And as I mentioned, the cut right through the rear threads is no small thing to contend with. Even with my equipment, it was challenging and much was still done by hand with Dremel & die grinders. Yeah, I could have made a new trunnion and receiver tube from scratch, but I've never tried welding up a torched kit, thought I'd give it a go. Time-wise, probably pretty close to a toss up.

If you buy one of the Horizontal cut kits with a milled trunnion, putting it together with an "80%" receiver tube would be pretty straight forward, probably doable with minimal tooling beyond a MIG or TIG welder, drill press and die grinder or Dremel if you don't mind the time.

 

They are good drum mags, I even converted one for a 9mm ar once.

 

Hey thanks for sharing. They are neat for sure. I remember someone showing off their 9mm AK that took Suomi drums, not sure how it ran but it looked awesome lol.