Green’s Dictionary of Slang

mop n.3

(UK Black) a machine pistol.

Perm ‘What's all the talk about’ 🎵 2 shells in the 12 / 6 in the spin / 30 rounds in the mop (whoosh).
67 ‘Jump Out Gang’ 🎵 [E]veryone runnin’ / But the corn just slapped and floored ’em / 50 shots in that mop.