While in Canada after the big shoot out, Ness picks up one of the gangsters whom had been shot on the porch of the old house. Ness sticks his gun in the corpses mouth and pulls the trigger. There is only a small blood splatter on the glass window behind the corpses head. A gun placed in someone's mouth and fired, the bullet would go through the head and exit out of the back of the head. There would be more blood splattered and the bullet would have exited the head and broke the glass. The velocity of a bullet would not have stopped before exiting the head. Especially when you think that the thickness of the skull between the back of the throat and the back of the head is not very thick and wouldn't most likely stop a bullet from exiting a skull.