Suomi M-31 SMG

The Suomi-konepistooli KP-31 (Submachine Gun Finland KP-31) was a submachine gun of Finnish manufacture that was in service during World War II. It was 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 just Suomi KP.

The Suomi KP-31 is regarded by many as one of the best submachine guns of World War II and it was so successful that some features from the Suomi KP-31 were copied in the design of the Soviet PPSh-41 submachine gun, such as the magazine. The accuracy compared to the mass-produced PPSh-41 is superior, thanks in part to a noticeably longer barrel, with the same rate of fire and an equally large 71 round drum magazine. If anything, it also possesses superior durability over the similar Russian SMG.

The M-22 and KP-26 were made by Konepistooli Oy (literally "Submachine gun company" in Finnish), founded by Master Armorer Aimo Johannes Lahti, Captain V. Korpela, Lieutenant Y. Koskinen and Lieutenant L. Boyer-Spoof. The Suomi-submachine gun KP-31 was designed by the duo of Lieutenant Koskinen and Master Armorer Lahti.

The Suomi KP-31 went into serial production in 1931 by "Tikkakoski Oy" and most of these weapons were bought by the Finnish Defence Forces. The Finnish Defence Forces were equipped with about four thousand Suomi KP-31 submachine guns when the Winter War started.

A variant with an underslung Flamethrower was produced but not much is known about this.

Suomi was also manufactured under license in Sweden, Denmark, and Switzerland. Tank and pillbox variants were also developed and produced in Finland.