Suomi KP / - 31 The Suomi KP / - 31 ( Suomi - konepistooli or " Submachine - gun Finland " ) was a submachine gun ( SMG ) of Finnish design that was in service 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 costs . 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 , which barrel shroud ls end is thinner and flattened to allow firing through the narrow ports of defensive bunkers . This version lacked a 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 gun s 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 , possibly because the Army forgot that they existed . The Suomi KP was also manufactured under licence in Sweden , Denmark , and Switzerland , where it was known as the Hispano - Suiza MP43 / 44 . 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 .