They did testing on what bullets do when they penetrate the human body as well. They fired into gelatin. Those bullets did the most remarkable dances you have ever seen. The only thing they didn't do is travel in a straight line.
That said, we may never really know what happened to Kennedy. I did see the documentary they were showing recently suggesting that Johnston killed him. I also think that it has absolutely no relevance to today.
Having treated a large number of gunshot wounds - this statement is absolutely wrong. The bullet begins to transfer a massive amount of kinetic energy the millisecond it strikes the body. The body also immediately begins to move or spin in the direction of the bullet's path.
Those bullets did the most remarkable dances you have ever seen. The only thing they didn't do is travel in a straight line
This is known as cavitation. The bullet creates a pressure wave inside the body. This wave of immense pressure results in an irregular path, massive damage to tissues away from the bullet's path, and upon exiting, a much larger exit wound. Exiting, however, does not cause a recoil.
I remember seeing the Nova episode you mention. I also remember laughing at the way the melons were positioned and shot - a lot of tricks with angular momentum. The melon spun like a top but still went backwards. Try it yourself - put a melon on a fencepost.
However, the dynamics of shooting a melon are nothing like that of the human cranium. Nothing - not in density, strength, etc.