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.

[8] Though a relatively early design, the Suomi was a formidable weapon: highly controllable[9] and with accuracy similar to that of the mass-produced PPSh-41[10] but at the cost of a significantly higher mass to absorb recoil, with a lower rate of fire and an equally large magazine capacity.

The Suomi KP/-31 went into serial production in 1931 by Tikkakoski Oy which was owned by German arms dealer Willi Daugs and most of these weapons were bought by the Finnish Defence Forces.

During the course of the war, the design was altered in February 1942 with the addition of a muzzle brake, which increased the submachine gun's overall length by 55 mm and weight by half a pound.

Aimo Lahti was displeased with this revision, believing that it decreased muzzle velocity and reduced the weapon's reliability, and even sought in vain to have the unknown designer of the brake court-martialed.

This was effected using a mitten or piece of thick cloth to secure, rotate, and remove the barrel jacket.

It had a removable front-plate that was quicker to reassemble and was cocked by rotating the mainspring for up to four detents, allowing the drum to be partially reloaded.

By the time of the Continuation War, Finnish doctrine had been altered to include both a KP/-31 and a light machine gun (usually a captured Degtyaryov) in every infantry squad.

[citation needed] It was sold to both sides during the Spanish Civil War; about 300 were confiscated by France from interned Republican troops in 1939.

It used a 56-round four-column "coffin" magazine designed by Linde AB and manufactured by the Carl Gustav State Rifle Factory.

The later MP43/44 had a fixed square notch rather than adjustable rear sights and the barrel jacket was modified to take a bayonet mount.

It had a quick-detachable barrel system and an arrangement whereby the spring was mounted inside the bolt in order to make the gun shorter.

The Russian company PUFgun makes a 5.45×39mm 60-round AK-74-compatible all-polymer coffin magazine that was based on an abandoned Izhmash prototype.

An even rarer version "900 kp 31 psv"[12] was produced for use as a secondary gun in the firing ports of Vickers Alt B Type E 6-Ton tanks, but only a few dozen were built before production was canceled due to the outbreak of the Winter War.