The Suomi KP/-31 (aka Suomi M/31), commonly known as Suomi submachine gun (Suomi konepistooli) in Finland, was a sub-machine gun based on the early Bergmann MP18 design. First designed in 1922, by Aimo J. Lahti, the gun was revealed to public in 1925. The Suomi (which means Finland) KP/-31 was in service in the Finnish Armed Forces from 1931 to the 1960's, and was also deployed in large numbers by other military forces during the Second World War. Some special versions stayed in service through the 1980s.
While somewhat similar in appearance and shape to the Soviet PPD-40 and PPSh-41, the KP/-31 is not functionally related to the two (The KP/-31 has a straight blowback action, while the PPD and PPSh-41 rely on an open-bolt system.) However, Soviet experiences against the KP/-31's 71 round drum magazine during the Winter War did inspire the drum mags that would later be used on their own PPD-40s and PPSh-41s; this is the only significant relation between the designs.
