A Shit Machine Gun is an explosion of shit from your ass when you bend over. It needs to have enough force to actually shoot, or fly, and not just fall, from your ass. The consistency is moderate; somewhat watery, somewhat chunky. Mild diarrhea is good for producing a powerful shit machine gun.
"Dude, I fired a wicked powerful shit machine gun the other day. It put a hole in the wall!"
"Yesterday I gave Sarah a shit machine gun and now she's in the hospital with a broken rib."