The Suomi KP/-31 (Konepistooli) was a Finnish submachine gun designed by Aimo Lahti and produced by Tikkakoski Arsenal. It served as the Finnish Army's standard submachine gun during the Winter War and World War II.
The KP/-31 was derived from Aimo Lahti's previous design, the Suomi KP/-26. It was tested by the Finnish Army and accepted into service in 1931, and in 1932 the patent rights for the weapon were secured by the state-owned armory at Tikkakoski, where full-scale production commenced. The KP/-31 was also offered for international sale, with customers including Sweden, Estonia, and Norway, who produced their own licensed copies of the gun. In 1936 the KP/-31 was investigated by the British, and although they did not adopt it, they concluded that it was one of the best submachine guns they had tested.
The KP/-31 first saw significant combat use in 1939, during the Winter War against the Soviets, where by all accounts it performed very effectively and gave Finnish troops a firepower edge over the Russians. The wartime production of the KP/-31 saw the introduction of a muzzle brake modification called the Suujarru (SJR), against the wishes of Lahti. Captured examples were examined by Soviet engineers, who borrowed design elements for their own submachine guns, such as the PPD and the PPSh-41.
During the Continuation War, the Finnish Army increased their issue of KP/-31s to ensure that at least one man in every infantry squad was equipped with the weapon. By 1943, this had changed to two men in every squad, and even then there was consideration for increasing the number further. However, production of the weapon proved too expensive and slow, and a cheaper submachine gun was sought. The Army settled on a clone of the Soviet PPS, produced at Tikkakoski as the M/44, which superseded the KP/-31 in Finnish service. Existing KP/-31s remained in Finnish service until the 1980s, albeit modified after the war to accept Carl Gustav M/45 magazines.
The KP/-31 was a blowback-operated submachine gun that utilized a hollowed-out conical bolt with a fixed firing pin. Housed in the rear of the bolt was the return spring. When the bolt retracted after firing, the protruding return spring would cushion the bolt's contact with the rear end of the receiver and push it back forward into the breech.
During production, the KP/-31 was machined from a solid piece and fitted with wooden furniture. Consequently it was very heavy but also sturdily built and not liable to break. It also ensured that the weapon functioned well in harsh conditions and was not very susceptible to snow or mud.
The various magazines designed for the KP/-31 were of considerable interest. Aside from a basic single-stacked box magazine, the gun was issued with a "casket" magazine, which fed rounds from two separated double-stacked columns, and a rotary drum magazine.
The SJR variant differed from the regular KP/-31 only in that it had a muzzle brake; otherwise it was identical.
A variant with a pistol grip in place of a full stock and an extended barrel. It was designed to be fired from bunkers or vehicle ports.
The M/37-39 was a licensed Swedish copy of the KP/-31, produced by Husqvarna Vapenfabriks AB. It had a shortened barrel and raised buttstock, and served as the Swedish Army's standard submachine gun during the 1940s.
The MP43 was a licensed Swiss copy of the KP/-31, produced by Hispano-Suiza on the request of the Swiss Army. The MP43 had simplified sights and bayonet fittings.