Suomi KP/-31

The Suomi KP/-31 (Finnish: Suomi-konepistooli m/31, lit. 'Finland-machine-pistol mod. 1931') 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.