The TNW M-31SA Suomi Carbine

For several years TNW Firearms of Oregon has made the M-31SA carbine, which is based on the KP-31 Suomi submachinegun adopted by Finland in 1931. The KP-31 gained famed during the Winter War of 1939 – 1940, and saw extensive use during the Continuation War of 1941 – 1944.

Click the pic for full size photo.

The KP-31 was a very influential weapon. Before being exposed to the Suomi, the Soviet Army didn’t employ SMGs in significant numbers. They were regarded as “gangster guns.” However, being on the receiving end of fire from Suomi SMGs wielded by Finnish troops changed Soviet leaders’ minds. For example, the drum magazine of the PPSh-41 is a nearly straight copy of the Suomi’s 71 round drum.

The specifications of the KP-31 are:

• Operation: Blowback, selective semiautomatic or fully automatic
• Weight: 10 – 11 pounds
• Cyclic rate of fire: 750 – 900 rounds per minute
• Barrel length: 12.6”
• Feed: 20 round box (“stick”) magazine, 40 round drum magazine, 71 round drum, or 36 round stick.

Specifications of the TNW M-31SA are:

• Operation: Blowback, semiautomatic only
• Weight: Approx. 12 pounds unloaded
• Barrel length: ~16”
• Feed: Same as the KP-31

Because the Firearms Owners’ Protection Act of 1986 amended previous Federal laws, no new fully automatic weapons for civilian, non-law enforcement ownership may be brought into the United States. Therefore, when Finland sold off their stock of Suomi SMGs in the 1990s, they had to be demilitarized into torch-cut parts kits before being sold on the US market.

18 U.S.C. Section 922r also restricts the number of imported parts that may be used in the construction of a new semiauto firearm for American civilian use, so when building new rifles from the Suomi parts kits, TNW had to use the following US-made parts:

• Receiver
• Hammer
• Trigger
• Disconnector
• Barrel

As a shooter with a background in military history and a long time interest in the Winter War, the Suomi has long held a fascination for me. TNW’s semiauto version caught my eye several years ago. Their primary online dealer, Centerfire Systems, has been out of stock for a little while now but the M-31SA remained available directly from TNW. Not wanting to miss my chance of owning this piece of history, a couple weeks ago I decided to pull the trigger (sorry) and order one.

A note on purchasing firearms over the Internet:

There has been much misinformation spread about buying guns on the Internet. I.e., calling it a “loophole.” In fact, unless you have a Federal Firearms License, the only way for you to buy a gun online is to have it shipped to a local FFL, who will then have you come in and complete the same paperwork and go through the same background check as if you go into a gun shop and buy something from their stock. The Internet merely gives buyers access to a wider variety and better prices. The local FFL will charge a fee for processing the paperwork. The FFL I use charges $30.

I already own a Suomi parts kit for another project, so I was already familiar with the quality of these guns as originally made. What was done to them was a crime from an historian’s perspective. They are a product of a bygone era. The metal parts were machined from large pieces of high-grade steel. The steel was given a nice, blued finish. The wood stock was well-crafted and fitted.

My initial impressions of the TNW M-31SA are as follows:

• Suomis are heavy sumbitches. Twelve pounds for a gun shooting 9mm Parabellum ammunition, by current standards, is excessive. But remember, the KP-31 was originally adopted 83 years ago. Manufacturing methods were a lot different.
• As a corollary to the weight, the Suomi feels solid in your hands. Holding the M-31SA gives you the impression that no matter the abuse that it suffered on the battlefield, a soldier armed with a Suomi could rely on his gun.
• TNW’s semiauto trigger mechanism gives a heavy trigger pull. Parkerized parts don’t help this. In fact, the trigger pull is more than weight of the rifle. It arrived bone dry and copious lubrication helped this, and I’m hoping it improves as the finish wears. I did some careful polishing inside the trigger group to improve it, however. I think I have figured out a way to reduce it further, and will do a follow up post with details once I’ve tried it.
• Instead of bluing, TNW parkerized the outside. This is a more practical finish in that it’s more protective but it isn’t as attractive.
• Other reviews have mentioned that TNW also parked the bolt and inside of the receiver tube. They should have left it bright, as with the originals. This would make the gun easier to cock and reduce break-in period. The bolt on my rifle appears to have been matte blued, which is an improvement over Parkerizing for this application.

TNW included with the rifle one 36 round stick magazine. This is actually a post-WW2 design, originally made for the Swedish m/45 Carl Gustav SMG. During the Winter and Continuation Wars, the Finns used 20 round stick, 50 round “coffin,” and 40 and 71 round drums with their KP-31s. The 36 round mag wasn’t adopted by the Finnish Army until the 1950s. It is one of the best SMG mags ever made. Because they are a double column, double feed magazine, it is easy to load. They are very well made. This 36 round stick was later used in the S&W Model 76 SMG, which itself was a copy of the m/45.

The 71 round drum is the magazine most associated with the Suomi. I have one, which required fitting to the rifle before it would latch in. As I understand it, the Finns individually fitted and serial number drums to their KP-31s, so this wasn’t a surprise.

The rifle was shipped with the barrel and shroud removed so the package would be shorter. Removing and installing these pieces is easy. To remove them, rotate down the latch that’s in front of the magazine well, then rotate the barrel shroud and pull it off. You can then pull the barrel straight out. Reinstalling these parts is simply the reverse of this procedure.

To remove the bolt and clean and lube the inside of the receiver, first remove the trigger group. This requires removing the screw behind the trigger guard. Then, with one hand pull the cocking handle back a couple inches and hold it, then unscrew the receiver cap. Make sure that you have a good hold on the cap because it’s under a lot of pressure. (Note: Early M-31SA’s made by TNW were striker fired and had two springs in the receiver. The later, hammer-fired guns have only one.)

Reassembly is basically the reverse but you need to be careful that the cocking rod doesn’t jump over the bottom of the bolt. Getting mine back together took a few tries. I don’t anticipate removing the receiver cap after each range session. I generously lubricated the bolt and it should be good for quite a while.

My rifle’s stock has evidently seen a lot of use. It was rough sanded and has several repairs. The sling swivel on the left side of the butt is in a different hole that it originally was. It looks as if the swivel somehow got torn from the stock at some point. The length of pull would be considered short by most Western shooters, but this is for good reason. It allows you to properly shoulder the rifle even if you’re wearing a thick winter coat. Given Finland’s climate, this is important. The stock from my parts kit is in better shape and has nicer grain, so I’m going to swap them.

TNW also included an original Finnish sling made from leather. It was in fair shape. I gave it a healthy coat of Sno-Seal to preserve it. The sling is a simple carry strap about an inch wide.

The front sight is a simple blade. It isn’t protected but it’s thicker than many other military blade sights and looks sturdy. The rear sight is an open tangent. The notch on mine was a little buggered so I cleaned it up with a file and widened the notch a little to make it easier to use with 45 year old eyes.

Today I took the TNW Suomi out to my friend's place to shoot it. We put 150 rounds through the gun with only a couple of issues:

1. After the first shot the trigger failed to reset. This may have been related to my tinkering with the trigger group yesterday in an effort to reduce the trigger pull. A nudge on it with my finger reset it, and this problem did not recur.
2. Two or three times we failed to properly seat the magazine and it fell out when we charged the gun. This is purely operator error and can be prevented by firmly seating the mag, then pulling on it to verify that it's latched in. Because I haven't finished fitting my drum magazine to the gun, we used 36 round stick magazines.

We shot from about 10 yards in my friend's yard. Recoil is like a .22 and the report isn't much louder. We put 100 rounds of Federal 9mm 115 grain FMJ from a Walmart bulk pack, and 50 rounds of Russian Brown Bear 115 grain FMJ with steel cases through the gun. There were no function issues other than what I mentioned above. It fed, fired, and ejected as expected.

SMGs may be obsolete for the military, but shooting this was just plain fun. Assuming it'll feed JHPs and continues to be reliable, it would work fine as a home defense gun, albeit a very heavy one.

If you’re interested in historic military arms, the Winter War, or Finland’s military history, the TNW M-31SA Suomi carbine is a fantastic buy.