Suomi M-31 SMG
| Suomi KP/-31 | |
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Suomi KP/-31
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| Type | Submachine gun |
| Place of origin | Finland |
| Service history | |
| In service | 1931–1998 Finland (in active service 1980s) |
| Used by | See Users |
| 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 |
| No. built | Approx. 80,000 |
| Variants | Kpist m/37, Kpist m/37-39, Kpist m/37-39F, Lettet-Forsøgs, Madsen-Suomi, Hispano Suiza MP43/44 |
| Specifications | |
| 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) |
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| 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 |
The Suomi KP/-31 (Suomi-konepistooli or "Submachine-gun Finland", literally: "Machine-pistol 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 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.
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