Sunday, May 3, 2015

Heavy Metal: The Suomi KP/31

Ah, the sub-machine gun. Much like the drive-in movie theater, the pay phone, and the typewriter, sub-machine guns are a useful, if not revolutionary item whose time has passed. As I mentioned when I wrote about the PPS-43, they're more of a curio these days than anything else, but as a military history and firearms enthusiast, they continue to interest me. The 1930s and 40s were the heyday of the sub-gun. Every major power on the globe used them, and many smaller countries as well. And with good reason! Sub-guns bridged that gap between a battle rifle and a pistol, a gap that only someone who has worked in a military support or leadership role knows so well. For your consideration, I present one such gun that excelled in its niche: the Suomi-konepistooli 31 (Finland automatic pistol 31).*

A beautiful specimen with some honest wear. Note the figuring in the arctic birch stock.

Perhaps you remember Aimo Lahti, the famous self-taught Finnish firearms designer. No? Then go read about the pistol which bears his name.  One of Lahti's most prolific and successful designs was the KP/31 submachine gun. An early-ish sub-machine gun design that was developed during peace time, the KP/31 was somewhat of a labor of love. At the time, Finland's submachine gun was the foreign produced Bergmann. Lahti didn't think much of it. He wanted to develop a cheaper, more reliable weapon on his native soil. Lahti developed a prototype in 1922, but could not get state financing to proceed further. Collaborating with other Finnish Army officers, he raised capital and established his own private company, where work began in earnest. Their initial result was the KP/26, a rather odd looking gun with a highly curved magazine, and chambered in 7.65mm Luger. The Finnish Army wasn't much interested in submachine guns at the time, and the KP/26 was far from perfect, so the military procured only a few examples. As such, the KP/26 never really saw combat, and was relegated to Civil Guard and home defense roles.

The weird KP/26. You can see the influences this weapon had on the later KP/31.

This KP/26 had some reliability problems and was finicky about interchanging magazines between weapons, neither of which are characteristics that are desired in a firearm meant for military service. So, Lahti and team went back to the drawing board. They retained things that worked well (like the quick removable barrel) and eliminated those that didn't, including the curved magazine and odd stock. Internal enhancements to the bolt cured the reliability issues, and with a caliber change to 9mm Parabellum, the KP/31 was born. This time the military paid attention. Lahti sold the production rights for his new gun to the famed Tikkakoski company, and Finland had its first, viable, mass-produced submachine gun in 1931.

KP/31s being built at Tikkakoski. Note the barrel jackets still in the white. Photo courtesy of

The KP/31 was a blowback operated, open bolt, select-fire weapon with a non-reciprocating charging handle, and it was a built like a tank (perhaps from a tank?). Stamped sheet metal parts? Surely you jest. This thing was forged, and parts were milled out of solid steel. Even unstressed parts like the barrel jacket were machined. Topped off with a hardwood stock of arctic birch, the KP/31 was no lightweight. It tipped the scales at nearly 10 1/2 pounds, empty. This was nearly a pound more than a  M1 Garand! Slap a loaded 72 round drum magazine into the KP/31 (and you should!), and weight increased another 3 pounds. Lahti had given birth to a heavy baby boy. The upside to all of this steel meant that the gun was extremely durable and built to last.

This young lady doesn't seem to mind the weight of her KP/31 variant (with coffin mag), which is possibly a Swiss export model. The gun, not the lady.

The Finns put the KP/31 to good use against the Soviets in the Winter War and Continuation War. The gun fired ~900 rounds per minute, and as mentioned, had a unique quick change barrel which made it capable of long periods of sustained fire. Initially equipped with 20 round magazines, the Finns quickly realized this was inadequate and adopted a 72 round drum magazine, and then an interesting four column 50 round "coffin" magazine. Standard load-out during the Winter and Continuation Wars was either five drums or seven coffin mags, giving a soldier quite a bit of firepower. Typically, one soldier per rifle squad was equipped with the KP/31, making it the squad automatic weapon of its day.

Patent drawings of the original 40 round (later 72 rd) drum magazine (left), and quadruple column "coffin" mag (right).

Ergonomically (other than the weight), the KP/31 excelled. It came naturally to the shoulder, the wood stock provided a solid cheek weld, and it had a longer barrel and sight radius than its contemporaries. The weight of the gun actually did have a positive impact: it minimized felt recoil, but reports from the field showed that muzzle rise could be a problem. To counteract this, the barrel jacket was given an integrated compensator. This added about 2 inches to the already long-for-its-kind 12.5" barrel. Reportedly, Lahti was vehemently opposed to this modification as he felt the compensator decreased muzzle velocity and hurt the gun's reliability. All told, the Finns produced roughly 80,000 KP/31s and associated variants (some of which, like the tanker and bunker models, were really cool), with a very small percentage being exported. The gun stayed in front line service until the 1960s, when it was replaced by an AK style assault rifle known as the RK.62. Still, KP/31s were kept as reserve weapons until the late 1990s. Because the damn things just worked.

The Finns loved their Suomis (center), but they weren't above using captured guns like the excellent PPS-43 (left). Safety note: placing one's hand over the muzzle of an open bolt sub-machine gun is an especially bad idea.

Thanks to a myriad of restrictive laws, it's extremely difficult and expensive for an average American citizen to own a fully functioning KP/31. Thanks to American ingenuity, however, the average American can own a pretty cool facsimile of one. In recent years, a firm called TNW Firearms procured a large quantity of demilitarized KP/31 "parts kits". These were basically KP/31s that had been disassembled, with the receivers torch cut per BATFE specifications, but the rest of the gun was largely left intact. Taking these parts and a newly manufactured upper receiver, TNW re-built the KP/31, turning it from an open bolt, select fire submachine gun into a closed bolt, semi-automatic carbine. Outwardly, the only real distinguishing difference between TNW's semi-auto Suomi, and the real deal, is the barrel extension welded onto the original Suomi barrel in order to bring it to NFA mandated 16" length. I acquired mine about two years ago, which was toward the end of the production cycle. That's right: TNW no longer makes these guns. I asked them a while back if they'd make more, and their response was that they would not, since parts kits with serviceable barrels and stocks are no longer available in quantity. The price? About $500, and that included a sling, one 72 round drum magazine, and a 36 round stick magazine (the 36 round magazines are post WWII issue, and they work quite well).

With the 36 round stick magazine. Note the barrel extension is only slightly longer than the barrel jacket. Worth SBR'ing?

Overall, TNW's rendition of the KP/31 is a nice piece of kit. The stocks are like new and all metal is freshly Parkerized in dark gray (the originals were blued). You can see where they did some welding work, but though it's visible it's not what I'd call sloppy. Function? Mine's perfect. Some early TNW guns were striker fired and reportedly had issues; mine is hammer fired. The trigger pull is about as heavy as the gun (no exaggeration here, I'd estimate it's a good 10 pound pull), but ignition is positive. My gun has the integrated compensator on the barrel jacket (so-called SJR version). Best of all, TNW retained the quick-detach barrel system. Meaning, this gun is a prime candidate for a short barreled rifle conversion: once you pay the requisite $200 tax and receive approval on your ATF Form 1, you can attach an original Suomi barrel in seconds. I've not done that to mine as yet, but may get around to it someday. From the shoulder at 25 yards, the gun is as accurate as you could want, and will make short work of any tin cans on the berm. Firing a 72 round drum from this position does get a little tiring, but you probably won't care because of how much fun it is. If you can still find one, and you're someone who appreciates history and military firearms, I highly recommend picking up this iteration of one Mr. Lahti's greatest designs. You won't regret it. 

The last known picture of Aimo Lahti, taken 3 weeks before his death on April 19, 1970. He lives on through the firearms he created.

*For a treasure trove of info on Finnish weapons, including the KP/31, visit, the main resource I used for this blog entry.


  1. Does anybody else cringe at the photo of the soldier covering the muzzle with the palm of his hand?

    1. Then again, that solider is Lauri Törni, a Finnish military legend who also served in SS and later in American special forces. He died in a helicopter crash in while doing his super secret stuff in Vietnam.

      I assume he knows what he's doing.