Building The Suomi M-31 Semi Auto
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The Barrel

When I first started working with the Suomi, there was a serious issue with the take down barrel system. At that time, the ATF was saying that if the gun needed a 16" barrel to be legal, and an original short barrel COULD be installed without modification, the gun was illegal. Now, following the Thompson Center Supreme Court decision, this is no longer true. You must now be either in possession of the rifle with the long barrel installed, or be in possession of ONlY long barrels. My advice is to not own a 16" barreled Soumi rifle and also own any short Suomi barrels unless you also own a pistol configuration Suomi.

If you're going to build your gun as a pistol or register it as a “Short Barreled Rifle”, you can ignore this section.

These instructions are describing the use of the Tommerr barrel modification. You are no longer required to do this. If you want to use the much simpler original method, just think in terms of fitting only one receiver prong to the barrel. The instructions for both systems are otherwise the same.

What you need to do is to first, very carefully, align the original barrel in the receiver with the top centerline of the barrel aligned with the top centerline of the receiver. You can use the original notch in the barrel to decide where top center is. This alignment is critical as the bolt's extractor must line up with the cut out in the back of the barrel.

(Tommerr mod only) I then scribed the location of the three new tabs on the recoil ring on the chamber end of the barrel. I next measured and recorded the width of each of the three receiver tabs. They were all different sizes to ensure proper barrel insertion. I carefully cut out the three slots in the ring. Using a center cutting end mill (that is slightly undersized for the smallest tab) in a drill press is a good way to do this. Make sure the barrel is solidly clamped and leveled in a drill vice so you don't break the end mill. Just bring the rotating end mill down slowly on the ring; use lots of oil and it works fine. Then hand file the edges of the barrel slots for a good tight fit. Go slow - be careful!

Next comes fitting the modified barrel into the receiver. This is a process of working with the shroud and the receiver together. You should have already fit the shroud and receiver to a 90+% fit and have the barrel slots cut for the alignment tabs. Now you can fit all three pieces to 100% fit.

This is a process of carefully fitting, by filing, the flat surfaces shown to the left. Work the barrel down evenly. Don't remove metal from the prongs, keep the original width.

You're setting the head space and the feed depth when you do this, so do it well!

Insert the barrel into the receiver and attempt to engage the shroud. See how far the shroud lacks reaching the receiver. This is how far the barrel needs to be set into the receiver. Don't try to do it all at once. Carefully measure the gap between the shroud and receiver ring. Coat the “nose” of the receiver on all sides with magic marker or layout fluid and scribe a mark this distance down from each flat that you are going to cut back. Use a hack saw to carefully cut back to “not-quite-to-the-mark”. From here on it's a file job. Use a bright light and magnifier to assess which flat needs to be filed a little more and test-assemble the parts often.

Patience is a virtue, mistakes are expensive! Take a break if it gets tedious.

You are done when the shroud will fully seat and twist the locking tab into the front filler slot. You had left it as only 90% finished previously. Work all of the parts into final position together for a precise fit. Keep everything well coated in magic marker or layout fluid and look for friction points. Work for a snug fit.

If you have a shroud with the long compensator, there's another step that will be needed to finish the shroud. When you finish extending your barrel, you will need to bore out the end of your shroud so the end of the longer barrel can poke through. This is lathe work and needs to be done carefully as a good fit with the barrel is needed for accurate shooting. I would bore the hole to 3/4 inch before doing the final turning of the barrel extension. This way a good snug fit can be obtained.

Here, you can see mine in the finished state. I drilled the extension to sort of match the original compensator. I also got the first hole a bit too close to the end of the compensator.

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