When all your friends catch you Scoping other guys asses all the time. Very shameful, but it does happen.
Guy#1: "Hey, stop checking out guys`s asses"
Guy#2: "Im not, I was..."
Guy#1: "Yeah whatever, butt rifle"
A fart that is one single loud explosion. A very loud and if not controlled properly, could hurt the shooter.
Damn Lenny, did you hear that butt rifle?