Bullets also do odd things when they hit bone. I'd imagine a bullet hitting the skull (bone close to the surface) would cause an almost explosive reaction, especially large calibre at lower velocity.
You're correct in saying that bullets behave in an unpredictable manner, especially once they pass through soft tissue and liquid, and then strike harder objects like bone, however the head is a different media than the rest of the human body, as the skull is right under a tight thin layer of skin. Ask any mortician, pathologist, or even soldier/cop/contractor/etc who has seen a lot of head shots, and invariably they will say the same thing: Nearly all entry wounds are small, almost as though they were done with a drill, while the entry wounds, if they exist, which is typical, are somewhat unpredictable depending on the ammunition type and velocity. The JFK Zapruder film is very, very typical of a small to medium size rifle bullet entry and exit wound, at least one that isn't a contact wound, but one from medium range.
I still think the best evidence I've seen is the recent work done regarding the Secret Service agent having an AD/ND and striking JFK in the head from about 9 or 10 yards away with an M16. Many people saw the M16, and it is in several photographs as well. Also, many people at ground level in the following cars smelled gunpowder strongly, something that wouldn't be possible with the winds that day from the book depository, or the fenceline by the grassy knoll. It also explains the time factor, as it is extremely unlikely Oswald could have fired 3 shots in such a short time for 2 hits, yet 2 shots for 1 hit anyone with some rifle skill can do. I'm not saying there wasn't a conspiracy of some kind, just that the shots could have come from the depository, as well as ground level from close behind the President as well.
I've seen guys who are better trained and have far, far more experience with small arms then the Secret Service make mistakes, especially under fire, stress, and when lacking sleep/rest. I also believe that the area, position, and trajectory of the 2nd shot to the head is a hard sell from an elevated position, where as it's an easy sell from ground level. This being the case, where could this other shooter have been, and not been easily seen after firing? The investigators recently tried to find a spot where this could have come from, and the only reasonable place is the following car.
I can see why they would cover this up at the time - It's a pretty horrible thing to admit that the best guards in the world failed, and not only failed, but also greatly contributed to their protectee's demise. Just think how horrible it would have been if that's what happened. Some dork gets a hit on the President, and then the SS plants an accidental round in his head. Think of how the rest of the world, particularly the Soviet Union would have viewed this. The USA would have seemed very weak to them, although this wouldn't have been the case IMO. That fact could have had all kinds of negative repercussions.
Anyhow, watch the recent special regarding the Secret Service AD/ND theory, it did more to convince me then anything else so far, even though I've always strongly believed shot 2 was an exit wound through the front of the head, not an entry. Like I said, having seen a few head shots with rifle and the odd pistol round, the entry wounds have never, not once, made several square inches of head explode on the entry side by my observation.
