Another coinage from Vietnam to denote full automatic fire (usually of the AR-15/M-16. The early M-16/AR-15's did not have burst triggers - hence the whole magazine of 20-30 rounds could be run off fast). Hardly ever were two weapons fired in cadence so a long rrrriiiipppp of automatic fire followed by others had a surreal sound like bars from rock and roll songs.
Infantry conversation after a firefight: Hear that? Did you hear that?? I swear to Christ with us cappin` charlie and charlie cappin' us I fuckin' swear to God I heard someone playin' "Louie Louie" in there. Also to describe the state of mind and fighting mode intended by the grunts: "Fuck that 'pick `yer shot shit...I'm goin' in on rock and roll...gonna pray and spray the motherfuckers"