I’d think that would depend a lot on
the type of shell, the angle it hit etc.
I’d also think that in some cases, the brain—being the consistency of jello about 70% set . . . could be pushed down a bit by a shock wave.
I think what you are referring to is the calvarium. The scalp is torn front to back but the calvarium seems in tact. I am sure the exit wound of the projectile in to occipital region is.....generous, and with gray matter.