M - 31 Suomi submachine gun The M - 31 ( Or KP - 31 ) Suomi ( Finland ) Submachine gun was one of the weapons designed by Aimo Lahti . Adopted by the Finnish army in 1931 , was manufactured until 1944 . Received widespread use in the Winter War , where its success would influence Soviet designs . Remained in use until the 1990s , several copies were bought by the armies of the world , including Brazilian police forces . Description . The Suomi ( " Finland " ) submachine gun was developed by Finnish arms designer Aimo Lachti in 1920 - 1930 period . First prototype was built by 1922 , and by 1926 Lahti produced a working weapon , chambered for then - standard Finnish army ' s 7 , 65x22 Luger / Parabellum ammunition . Definitive version of the Suomi submachine gun was adopted by Finnish Army in 1931 as Suomi - KP Model 1931 , or simply KP - 31 ( KP stands for Konepistooli - Automatic Pistol in Finnish language ) . Suomi submachine gun was manufactured by Finnish company Tikkakoski Oy , and licensed to Denmark ( Madsen ) , Sweden ( Husqwarna ) and Switzerland ( Hispano Suiza ) . Used mostly by Finnish and Sweden armies , it was also widely exported into Baltic countries , some European and South American countries . Suomi was used with great success during Winter War of 1940 against Soviet Union , when , wisely used , this SMG showed to the world the importance of the submachine guns to the modern warfare . Manufacture of the Suomi was ceased in Finland in 1944 , but it was used well until the 1990s , when finally rendered obsolete and replaced in Army by assault rifles . Suomi submachine gun is a blowback operated , selective fire weapon . It fires from the open bolt , and used so called " differential locking " or " advanced primer ignition " principle , when fixed firing pin ignites the primer BEFORE the bolt stops on its way forward into the battery , so the bolt momentum of inertia is used to lock the chamber during the initial phase of shot , when pressure in the chamber is high . The bolt and receiver were machined from high quality steel and bolt was fitted to the receiver almost airtight . The rear cover of the receiver was screwed on to it also almost hermetically . This was necessary to achieve a fire rate reduction by using a simple vacuum valve in the receiver cover - when bolt moved back , the valve let the air out easily from the space behind the bolt . When bolt started to move forward , the valve closed itself , so difference of air pressures behind the bolt and in atmosphere slowed the bolt on its way forward into the battery . The charging handle was somewhat similar in appearance to one found on bolt action rifles ; it is located behind and below the receiver , and does not move when gun was fired . The safety - fire selector is located at the front of triggerguard , and gun could be fired in semi - auto or in full - auto Another interesting feature was the quick - detachable barrel and barrel jacket . This feature , more adequate to machine guns , was a welcome during intensive fire - fights , when many hundreds of shots were fired in fully automatic mode . Sometimes the barrel was also fitted with machined muzzle brake / compensator . Suomi was fed from box or drum magazines . Box magazines were conventional staggered - column ones for 20 rounds or twin - staggered - column magazines for 50 rounds each ( also known as " Coffin magazines " due to their shape , these could be described as two staggered - column magazines clipped together and having common cartridge exit ) . Drum magazines held 40 ( rarely encountered early versions ) or 71 rounds and later inspired Russians to adopt drum magazines for their PPD and PPSch SMGs . In mid - 1950s Finnish army also adopted 36 - round magazine , designed in Sweden for M - 45 Carl Gustaf submachine gun , and Suomi submachine guns were slightly modified to accept those magazines . In general , the Suomi KP - 31 was a highly effective , reliable and accurate gun , but too expensive to manufacture .