Look at the Zapruder film, Bud.
You say "a shot from behind will also make the physical matter go out from the back of the head."
The physical matter does not go out the back of his head. It sprays forward.
By your own reasoning, that bullet came from the front.
LOOK AT THE FILM.
BACK, AND TO THE LEFT. BACK, AND TO THE LEFT. BACK, AND TO THE LEFT.
Think Mother Teresa and Richard Nixon. Forget about the Magic bullet and the pristine bullet. The answer is in New Orleans, not Dallas.
Check the sewars, don't look to the left.
:-X