The Suomi KP/-31 ( or "Finland-submachine gun mod. 1931") is a submachine gun (SMG) of Finnish design 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<ref>Combat Guns: An Illustrated Encyclopedia of 20th Century Firearms, Osprey, 1987 .</ref> 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. Its 71-round drum magazine was later copied and adopted by the Soviets for their PPD-40 and PPSh-41. Though a relatively early design, the Suomi was a formidable weapon: highly controllable and with accuracy superior to that of the mass-produced PPSh-41 at the cost of a significantly higher mass to absorb recoil, with the same exceptionally high rate of fire and an equally large magazine capacity. Its one major disadvantage was its 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