Never saw a man being shot in a head, but the third and fatal shot sure looked like coming from the right front (I'm talking about the movie). It looks like his head goes vilolently back and top of his head is blown off to the rear. Jackie climbs back on a trunk to retrieve it.
As I said, just an opinion, but I know physics. I'm not buing the shot from the rear.
The fact that Oswald was shooting, does not mean that he was a the only shooter, not that he was a killer.