The Suomi-konepistooli, more commonly known as the Suomi KP/-31, is a Finnish submachine gun.
The weapon is the offspring of the older Lahti M-22 and Suomi KP/-26 submachine guns. The KP/-31 was first manufactured in 1931, with about 4000 being used by the Finnish Army. Midway during its production run, the KP/-31 was altered with a muzzle brake; however, Aimo Lahti, the designer of the weapon, was not at all pleased with this revision of the weapon, as he felt that the muzzle brake reduced the reliability of the weapon and decreased the muzzle velocity. He was so displeased with this revision, that he even sought (and failed) to have the designer of the muzzle brake for the KP/-31 court-martialed. Originally sought as a substitute for a light machine gun, the KP/-31 proved inadequate in the role, causing soldiers to try to learn how to use a submachine gun through trial and error. This had all changed by the time of the Continuation War, when Finnish doctrine was altered; the alteration stated that a light machine gun, usually a captured DP machine gun, and a submachine gun be in every infantry squad. By 1943, this had changed to 2 submachine guns per squad; there was even an intention of having 3 submachine guns per squad in 1944, but before that was put into action, the Continuation War ended and the plan was shelved.
The KP/-31 incorporated some new design features, such as the spring being mounted inside the bolt to reduce the weapon's length. The KP/-31 was one of the first weapons to use casket magazines, which were quad-columned; similar magazines were later used on the HAFDASA C-4 and AK-74.
- SJR (Suujarru)
Variant with a muzzle brake.
- Bunker variant
Shortened variant for use in bunkers.