Green’s Dictionary of Slang

(UK Und.) an Ingram Mac-10 machine pistol.

1999		Guardian G2 9 Sept. 8: The tabloids call them ‘Big Macs’; the gangsters call them ‘raincoats’.
2000		J.J. Connolly Layer Cake 291: ‘You leave anything inside, Jay?’ ‘Some bedding stuff, my phone and a Mach-10.’ ‘A big Mac! Fuckin’ ’ell.’.