Gerry,
If you watch all the simulated "melon" shooting you see in most instances that the melon explodes out the front, the the back part of the melon goes backwards.
Imagine a bullet that explodes when striking something. It is moving so fast, 3,300 ft/sec, it enters the head, travels some distance and then explodes. Let's say it travels 6 inches and explodes, remember, 3,300 ft/sec, it doesn't take much for it to travel the 6 inches. Now where is the bullet when it explodes? It's just below JFK's front right forehead and it explodes. It blows in all directions; blowing that area of JFK's skull out and away and also blows backwards thus pushing JFK's head back and to the left.