I went out recently on a permission with lots of rabbits, during the course of the shoot I took head shots, some to the side, front, and some to the rear.
All where dead instantly, side & front all twitched, jumped, and back legs going, the ones on the rear of the head, stone dead, just dropped no movement or anything! 2 still had grass in their mouth.
I presume it must be due to hitting a different part of the brain?