Suomi KP/-31

The Suomi KP/-31 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.

Suomi KP/-31
Class Manportable
Type Machine Guns
Manufacturer Tikkakoski (company)
Production Period 1931 - 1953
Origin Finland
Country Name Origin Year
Finland 1921
Country Name Operational Year Retirement Year
Finland 1939 1940 View
Finland 1944 1945 View
Korea 1950 1953 View
Palestine 1948 1949 View
Spain 1936 1939 View
ManufacturerName Production From Production To Quantity
Tikkakoski (company) 1931 1953 80000 View

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 5.45×39mm 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.

Type Submachine gun
Place of origin Finland
Service history
In service 1931–1998 Finland (in active service 1980s)
Used by Bulgaria, Denmark, Estonia, Finland, Israel, Nazi Germany, North Korea, Norway, Poland, Republic of Korea, Slovakia, Sweden, Switzerland, Soviet Union, Vatican, West Germany
Wars Chaco War, Spanish Civil War, Winter War, World War II (or the Continuation War), Lapland War, 1948 Arab–Israeli War, Korean War (Limited)
Production history
Designer Aimo Lahti
Designed 1921
Manufacturer Tikkakoski
Produced 1931–1953
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)
Cartridge 9×19mm Parabellum
Action Straight Blowback
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

End notes