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|Place of origin||Finland|
|In service||1931–1998 Finland (in active service 1980s)|
|Used by||See Users|
|Wars||Winter War, World War II (or the Continuation War), Lapland War, 1948 Arab–Israeli War|
|Number built||Approx. 80,000|
|Variants||Kpist m/37, Kpist m/37-39, Kpist m/37-39F, Lettet-Forsøgs, Madsen-Suomi, Hispano Suiza MP43/44|
|Weight||4.6 kg (10.14 lb)|
|Length||870 mm (34.3 in)
925 mm (36.4 in) (SJR)
740 mm (29.1 in) (bunker version)
|Barrel length||314 mm (12.4 in)|
|Rate of fire||750–900 rounds/min|
|Muzzle velocity||396 m/s (1,299 ft/s)|
|Effective firing range||200m|
|Feed system||20, 36 or 50-round box, 40 or 71-round drum. Also modified German MP 38 u. 40 32 round box magazine|
|Sights||Front blade, rear notch|
The Suomi KP/-31 (Suomi-konepistooli or "Submachine-gun Finland") was a submachine gun (SMG) of Finnish design used 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 Suomi KP.
The Suomi KP/-31 is regarded by many as one of the most successful submachine guns of World War II, also the soon developed 71-round drum magazine was later copied and adopted by the Soviets for their PPD-40 and PPSh-41 submachine guns. The accuracy of the Suomi was superior to that of the mass-produced PPSh-41, thanks in part to a noticeably longer barrel, with the same rate of fire and the equally large magazine capacity. The major disadvantage of the Suomi KP/-31 was its high production cost.
The Suomi KP/-31 also incorporated a few new design features, including an arrangement whereby the spring was mounted inside the bolt in order to make the gun shorter. Its 50-round quad-column "Casket" box magazine was more reliable than the early 40-round "bullets loaded nose down" drum magazine, and similar applications were used on the Argentinian C-4 submachine gun and present-day 60-round 5.45x39mm AK-74 compatible magazines.
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.
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 4000 Suomi KP/-31 submachine guns when the Winter War started. During the course of the war, the design was altered with the addition of a muzzle brake, which increased the submachine gun's overall length by 55 mm. The revised version was designated KP/-31 SJR (suujarru, or "muzzle brake"). 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 muzzle brake's designer court-martialed. Ultimately, roughly half of the KP/-31s in Finnish service were of the SJR version. Initially the KP/-31 was issued as a substitute for a light machine gun, and proved inadequate in this role. Instead, soldiers learned by trial and error how to use submachine guns to the best effect. 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 DP) in every infantry squad, and by 1943 this had been expanded to two KP/-31s per squad. KP/-31 production continued with the intention of adding a third submachine gun to each squad, but this plan was shelved in 1944 when the Continuation War ended.
A specialized bunker version was also produced in very small numbers (a total of 500 built) in 1941, the barrel shroud end of which was thinner and flattened to allow firing through the narrow ports of defensive bunkers. This version had no shoulder stock and was equipped with a pistol grip. An even rarer version "900 kp 31 psv"  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. Production never resumed, as captured Degtyaryov DP machine guns proved far superior in this role. Like the bunker version, the tank version had a pistol grip and no buttstock, and it could be quickly removed from the tank and fitted with a standard barrel shroud for infantry use if needed. The tank version remained in the Finnish Army's inventory through the 1980s, despite the tank it was designed for being retired in 1959.
In 2009, a semi-automatic version of the Suomi KP was produced for civilian sale in the United States, replacing the receiver and lengthening barrel to meet the standards of the National Firearms Act.
A Suomi submachine gun is the weapon used by a mass murderer in the Sjowall/Wahloo Swedish mystery novel The Laughing Policeman. In the novel, a character asks if the weapon is the Finnish version or the Swedish-licensed version. A detective says a Finnish-made version, though he uses the Swedish model number (m/37) in mentioning it.
- Bulgaria: 5,505 were bought from 1940–1942.
- NDH: 1,250 were bought from 1942–1943.
- Denmark: Madsen and Hovea made at least 1,400 M/31 copies designated M/41 derived from the Lettet-Forsøgs submachine gun.
- Estonia: 485 purchased in 1937.
- Nazi Germany: 3,042 were ordered by Germany from Finland during The Second World War and likely issued to the Wehrmacht and SS. Also 120 Suomi KP's were presented to the German troops of AOK Norwegen in 1942 for use on the Finnish Front. Most of these weapons left Finland with German troops in 1944 and were subsequently used in other theaters of the war.
- North Korea: Used along with weapons supplied by the Soviet Union during the Korean War.
- Norway: Norwegian police troops trained in Sweden used the Swedish m/37-39 variant. Used by the Norwegian armed forces after the war until the 1980s, primarily by the Navy and the Home Guard.
- Poland: Used by Polish Police (Policja)
- Republic of Korea:Captured by Korean People's Army limited, used during at the Korean War.
- Sweden: 35,000 made under license by Husqvarna Vapenfabriks AB just before and during World War II, some were also imported from Finland. Designated kpist m/37 (9mm Browning Long caliber) after the year of adaption, in 1939 Sweden decided to switch to 9 mm Parabellum and the new weapons were designated kpist m/37-39.
- Soviet Union: Used captured weapons.
- West Germany
Swiss MP 43/44.
Finnish soldier armed with a Suomi KP and a Panzerfaust, walks past a destroyed Soviet tank.
Media related to Suomi M31 at Wikimedia Commons
- "Gunwritersin Suomi-KP, osa 5". Guns.connect.fi. Retrieved 2011-12-06.
- Combat Guns – An Illustrated Encyclopedia of 20th Century Firearms"OSPREY, 1987
- "TNW Firearms, Inc. Manufacturers of the semi-auto 1919, M2HB and MG34". Tnwfirearms.com. Retrieved 2011-12-06.
- "kpistar i Sverige". Gotavapen.se. Retrieved 2011-12-06.
- Guns of the Third Reich, p.163
- Jäger Platoon
- Modern Firearms: Suomi
- The Finnish Weapons Series Konepistooli m/31 Suomi
- Suomi-KP History