Green’s Dictionary of Slang

(US) an automatic pistol or machine gun .

1991		A Tribe Called Quest ‘Everything Is Fair’ 🎵 Pullin out the ooh wop, listenin to doo-wop.
1995		S. Moore In The Cut 63: There seem to be more synonyms for guns than vaginas on this list. [...] oo-wop, gun.
1997		The Roots ‘The Roots is Comin’’ 🎵 Organix [album] The Roots, is out to blow up like a clip from out the milli or the oo-wop.
2011		G. Hayward Corruption Officer [ebk] cap. 16: These fools done fucked around and let a nigga purchase a firearm! It’s a gat, a ratchet, a biscuit, a burner to all my inner-city home boys and a ooohhwop to anybody over fifty.
2012		‘Dutch’ ? (Pronounced Que) [ebook] Muhfuckas had AKs, ow-wops, the whole nine!