Designed by Finland’s most notable arms inventor, Aimo Lahti, the m/31 Suomi submachine gun is an iconic weapon of the Winter War and the Continuation War. It is a first-generation submachine gun with a heavy milled receiver and very well-fitted parts – enough so that a series of vent holes were put in the end of the receiver tube to prevent air compression during the firing cycle. The gun is overall quite heavy (4.7kg / 10.4lb) and has a high rate of fire (~900 rounds/minute). It originally used 50-round quad-stack magazines and 71-round drum magazines, which allowed a shooter to exploit the high rate of fire without constantly having to change magazines.
The quad-stack magazines were eventually found to be insufficiently reliable, and were retired. The drum proved to be quite good in use, although quite awkward and bulky to carry (not that Finnish troops had no specialized pouches for these drums). The drum design would be taken by the Russians (handed over by a defecting Finnish officer, apparently) and copied for use in the Soviet PPD and PPSh submachine guns. In the 1950s, a simple double-stack 36 round magazine was developed and became most popular.
We also have a live firing assessment of the m/31 Suomi on InRangeTV: