We were provided access to quite a few awesome Finnish firearms while in Finland and we jumped on that opportunity to do analyze the M31 Suomi, but then also to compare it against the later Finnish SMG, the KP44.
The M31 is frequently referred to as the best sub machine gun of WW2, and there's good reason fo this - the Finns used them with telling effect in the Winter War. So much so, in fact, that the Russians landed up relaunching renewed SMG programs in Soviet military (which they had previously shelved). So, via a few degrees of separation, you can make the argument that the Finnish success with the M31 ultimately culminated in the development of the Kalashnikov!
Live fire footage and experience with the M31 is a frequently requested topic, so hopefully you enjoy this discussion of the M31 and our comparison of it the KP44!
Don't forget to also check out Forgotten Weapon's video regarding the M31 SMG: https://www.youtube.com/watch?v=8F4bjxmrLw4
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