The Suomi KP/-31 (Finnish: Suomi-konepistooli m/31, lit. 'machine pistol Suomi m/31') is a Finnish submachine gun that was mainly used during World War II. It is a descendant of the M-22 prototype and the KP/-26 production model, which was revealed to the public in 1925. It entered service in Finland in 1931, and remained in use until the 1980s. The Suomi KP/-31 is regarded by many as one of the most successful submachine guns of World War II. It also had a profound effect on the conflict beyond Finland as the Soviet authorities, who had been dismissive of submachine guns, were persuaded of their fatal efficiency by the Finnish forces in the Winter War of 1939–1940. Though a relatively early design, the Suomi was a formidable weapon: highly controllable and with accuracy similar to that of the mass-produced PPSh-41 but at the cost of a significantly higher mass to absorb recoil, with a lower rate of fire and an equally large magazine capacity. Its major disadvantages were its higher weight, high maintenance requirements and high production cost, which led to the later introduction of the KP/-44, a close copy of the Soviet PPS-43 but accepting the existing magazines and drums for the KP/-31. The M-22 and KP/-26 were made by Konepistooli Oy, founded by Master Armorer Aimo Lahti, Captain V. Korpela, Lieutenant Y. Koskinen and Lieutenant L. Boyer-Spoof. The Suomi KP/-31 was designed by Koskinen and Lahti.
History The Suomi KP/-31 went into serial production in 1931 by Tikkakoski Oy which was owned by German arms dealer Willi Daugs and most of these weapons were bought by the Finnish Defence Forces. The Finnish Defence Forces were equipped with about 4000 Suomi KP/-31 submachine guns when the Winter War started. During the course of the war, the design was altered in February 1942 with the addition of a muzzle brake, which increased the submachine gun's overall length by 55 mm and weight by half a pound. The revised version was designated KP/-31 SJR (suujarru, or "muzzle brake"). Aimo Lahti was displeased with this revision, believing that it decreased muzzle velocity and reduced the weapon's reliability, and even sought in vain to have the unknown designer of the brake court-martialed. Ultimately, roughly half of the KP/-31s in Finnish service were of the SJR version. The KP/-31 was unusual in that it had a replaceable barrel secured with four lugs rather than threads. Soldiers were issued at least one spare barrel. In close combat the weapon would overheat in sustained automatic fire, requiring the barrel to be changed. This was effected using a mitten or piece of thick cloth to secure, rotate, and remove the barrel jacket. Then the user would use the point of a puukko knife or a cleaning rod to loosen and remove the hot barrel. It was easy to then slide in and lock a fresh new barrel, secure the jacket, and resume firing. The Finnish military initially issued it with the 20-round staggered-column magazine and early 40-round sissilipas ("ranger magazine") drum. The magazine's capacity was found to be too small for sustained fire. Up to 5 additional rounds could be loaded into it, but it was found that this damaged or over-compressed the springs and caused it to fail. The drum was found to be hard to reload in action because the backplate had to be taken off and the bullets loaded inside tip-downwards. The drum also had an open "shot count window" in the bottom of the drum; the numbers 1 to 40 were stamped on the feed plate and lined up with the window to indicate the shots remaining. However, it naturally let in dirt and debris that led to jams. Aimo Lahti experimented with a 60-round Thompson-type spring-loaded clockwork drum to replace it, but it was never adopted. They were later replaced during the Continuation War with the Swedish-designed 50-round "coffin" magazine and improved 71-round drum. The "coffin" magazine carried more ammunition than the box magazine and was lighter than the 40-round drum. However, it was complicated and prone to jamming or failure if it was damaged, reassembled incorrectly, or the springs gave out. It would also fail to fire or feed unless it was properly seated in the magazine well. The "coffin" magazine was withdrawn from service in 1943 in favor of the 71-round drum. The larger drum was a design by the weapon's less-well-known co-designer, Lieutenant Y. Koskinen, that was an improvement of the 40-round drum. It had a removable front-plate that was quicker to reassemble and was cocked by rotating the mainspring for up to four detents, allowing the drum to be partially reloaded. It was first released commercially in 1935 (eponymously named the M/Koskinen drum) and entered into Finnish service in January, 1936. In the 1950s the magazine wells on the KP/-31s were modified in order to use the Swedish Carl Gustav m/45b's 36-round double-column magazine.
Tactics Initially the KP/-31 was issued as a substitute for a light machine gun, however, it proved inadequate in that role. Instead, soldiers learned by trial and error how to use submachine guns to the best effect. By the time of the Continuation War, Finnish doctrine had been altered to include both a KP/-31 and a light machine gun (usually a captured Degtyaryov) in every infantry squad. By 1943 this had been expanded to two KP/-31s per squad. KP/-31 production continued with the intention of adding a third submachine gun to each squad, but this plan was shelved in 1944 when the Continuation War ended.
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