Ok, maybe I'm too late? But, I have the oldest trick in the book for head shots. Show the gunman firing, cut to the victim(shown facing the camera) with a piece of meat and blood already taped to his forehead, have the compressed air hit taped to the back of his head and let her rip! Have the actor react to the impact and fall to the ground. You've got a nice juicy head wound and blood spraying out the back for the exit wound. With the right editing you can fool the audience everytime.