Green’s Dictionary of Slang

(UK black/gang) a weapon.

2012		Guardian 27 Mar. 🌐 He bragged [...] about how he was planning to ‘fuck up a couple Pagans’ (members of rival gangs) and later how he was going to get his ‘swammies’ (guns).
2017		Harlem Spartans ‘Kennington Where It Started’ 🎵 Swammies start blasting.