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The Suomi KP/-31 (Suomi-konepistooli or "Finland-submachine gun", literally: "Finland Machine-pistol") 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. The Suomi-konepistooli KP/-31 is often abbreviated to Suomi KP.
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 that conflict beyond Finland as the Soviet authorities, who had been dismissive of sub-machine guns, were persuaded of their deadly efficiency by the Finns after they attacked them in 1939. Its 71-round drum magazine was later copied and adopted by the Soviets for their PPD-40 and PPSh-41 submachine guns. 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 thanks in part to a noticeably longer barrel, 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.