And why did the bullet inside JFK's head explode when the MB didn't?
The twofer didn't hit large bones directly, just flesh. FMJ ammo was designed to remain intact while traversing flesh (as per the Geneva Convention).
The headshot hit the hard bone of the skull nose-first.
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A Pressure Cavity Blew Kennedy's Head Apart
Cite: John MacAdamsThe simplest assumption about the explosion of Kennedy's head is that an exiting bullet ripped a large defect and the bullet and brain matter exited at the defect. In fact, the process was a bit more complicated. When a bullet hits human tissue, it loses a lot of energy in under a millisecond, and that energy has to go somewhere. In fact, the energy creates a pressure cavily in the tissue ? almost as though a small stick of dynamite had been set off within the body.