In The Beginning ...
The project you're about to embark on may be one of the most detailed and complicated projects you've done to date. The Suomi M-31 is not the easiest parts kit to complete. However, it's a classic and a worthwhile addition to any collection.
The Suomi was a very well designed and innovative firearm in its day. Many experts in the 1930s and 1940s considered it to be the best sub machine gun ever built. Its biggest downfall was its quality. It was just too well made and expensive to produce. Had Finland not been aligned with Germany at the start of World War II, England would most likely have adopted it as their standard SMG!
Two problems that will surface immediately if you try to rework the original bolt, are its hardness and the fact that you need both a lathe and a mill. The bolt was hardened to a 55 to 60 Rockwell level. This is as hard as a file! To re-machine an original bolt requires either top grade coated carbide cutting tools, or to professionally anneal the bolt to a workable hardness. This annealing must be followed later with re-heat treating the bolt. Contact Prexis/Sten for these services.
Of course, the new Prexis/Sten already completed bolt makes life a lot simpler! It also adds one important 922r USA made part to the parts count! For those with the time and tools and a desire to use the original parts as much as possible, I'll go through what I did to re-machine my first Suomi bolt (the Prexis bolt wasn't available yet). Or, you can have someone else do the specialized machining.
To complete the Suomi receiver yourself, you do not need a mill or a lathe. A mill would prove very valuable and there's always the option of having others do some of the machine work. However, you can do almost anything on the receiver with a drill press, files and a Dremel tool that can be done on the big machines. It just requires patience, thought and an eye on safety.
Please note - you MUST complete the receiver yourself. Either that or have it completed by a licensed gunsmith with a manufacturers license. Only a licensed manufacturer can make a firearm for someone else (a receiver IS a firearm to the feds). It's federal law! As I said before - KNOW THE LAW!
Before you get too far into this, a key issue is the barrel length. This decision is based on what you're trying to build. You have two choices here. Are you building a pistol or a rifle?
Normally, cutting a rifle/carbine down to a pistol will get you into all kinds of trouble and even some jail time! However, you're in a unique position here with this parts kit build. The key is the term “parts kit”. When the BATF made the importer flame cut your receiver into a parts kit, they converted a firearm into a non-gun. Your parts kit in its original form, is not a gun.
Also, your Prexis/Sten 80% receiver is not a firearm - yet. When you finish machining it, it magically becomes a legal firearm. As you're the builder, it's totally your choice as to what you want to call it. In more ways than one!
You may declare it a pistol receiver or a rifle receiver. You can also name it! I recommend that you do just that. As the Nazis in Washington DC like to ban guns by name, it may be a good idea to stamp your receiver as a "Smith M-31"or some such so it won't ever show up on a Gun Ban list.
If you decide to make it a rifle, then you must insure that the barrel is over 16 inches in length. Otherwise, it's a Short Barreled Rifle which requires a $200.00 federal tax stamp. Not only that, some states have laws flat out banning the possession of a SBR. Therefore, you must do something about the fact that the Suomi has a 10.5 inch barrel. One choice is fit a new American made 16+ inch barrel. Another is to weld an extension onto the original.
If you decide to build a pistol, the existing barrel is fine, but you have to cut off the stock and fit a pistol grip. Also, as a side note, if you build a pistol, you can ignore all the 922r parts count stupidity. See the side bar menu for 922r info.
If you're making a rifle, there's an issue with the existing Suomi barrel takedown system. The BATF doesn't like it when you build a rifle, but can easily stick a pistol length barrel into it. So you must either weld the barrel or shroud to the receiver, so the barrel cannot be removed, or modify the existing system so an original short barrel can't be used.
This photo shows an original barrel, an extension tube and an alignment rod used to hold them in line for welding. About 3/4 of an inch of the muzzle end was turned down to .500 (lathe work) and the tube was bored to slightly over that dimension. The rod was turned to be a snug slip fit in the bore, and also to fit snugly in the extension tube. Good alignment makes sure that bullets won't graze the edge of the tube on the way out.
Make sure you make the rod long enough that you can grasp it to pull it out after welding!
Another option is called the Tommerr Mod.(Tom Radant) It should be ordered on your Prexis/Sten receiver to start with, but you could do the entire thing yourself if you already have a receiver without it. In the following photo you can see that instead of the single barrel locating projection, a receiver with the Tommerr Mod has three (on top surface) and they are all in a different location than the original. This prevents the simple dropping in of a stock barrel. NOTE - this modification is no longer required by the BATF!
When I built my first Suomi, I had to go with the Tommerr Mod, so I will spend more time discussing how I did this later.