These were very much in usage by the Wehrmacht during WW2. My uncle john, who served directly under Gen Patton
during WW2, called this the "burp" gun, as the GIs were wont to do.
https://www.dday-overlord.com/en/material/weaponry/mp-40-schmeisser He said they fired so rapidly that they sounded like a burp, rather than individual shots. effective in close combat, but must have been tough to keep supplied with ammo.
Not greatly unlike our M3 "grease gun", in that they both operate from a "open bolt' setting. However, the M3 is mnuch slower cycling rate (450 rpm).
