Head shots

How much punishment can the human skull take? I was watching a movie where both protagonists took head shots from the head of the other guy, boom!..forehead to forehead…and both guys jumped back up to continue the fracas. I watched a head butt in real life, and, just the sound of it was enough to make you wretch. Needless to say, the real life incident didn’t allow the hit-ee to get up off the floor. Are all adult skulls the same thickness and strength? If I was to head butt someone, would it behoove me to use the top of my head, or my forehead? Are some people capable of using a tactic like this in a fight, because their skulls are stronger?
Inquiring heads want to know.

Dunno about stronger skull, but I doubt even Iron Mike could take a proper headbutt.

Use the part of your head just above the hairline. Keep the neck rigid, bend the knees and drive in using the power of the legs. The reason for not using the forehead is that firstly it doesn’t allow correct technique. The other reason is that if you swing back a trained opponent will drop his head. Your forehead will collide with the front of his scalp. You both lose, but you lose worse.