I'm guessin' a .50 or, at the very least, a hollow point .30 caliber.
Leanin' toward .50, though.
That's an awful lot of damage to the brain pan.
Having a 50cal myself it looks like it but it would also get people behind him too. The 30.06 7.62 x 54r many others would do it, as you say with hollow point bullets. He never knew what hit him