Having fired the Mp40, PPSh 41 and PPS43 under reasonably rigerous conditions my vote goes for the PPSh 41. I fired a weapon that spent 40 years in a bog, was dug out, cleaned up and had no problems emptying several mag's under pretty serious weather conditions (-15 degrees C without wind chill). When I fired an MP40 in the UK in spring it jammed several times, easy to clear but still a pain in the backside, the PPS43 was similar (in fact, I think I may be the only person in history to break one by cocking it too hard).

That said, SMG's don't win wars, men, blood and bigger weapons do.

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There's no honorable way to kill, no gentle way to destroy. There is nothing good in war. Except its ending.