i would aim for the medulla. but even if i hit the pons or circle of willis i'm also dead. imo with the most powerful magnum slugs or rifle ammunition even from 2 inches away the shockwave would kill the brain stem or arteries supplying the brain stem like the circle of willis and other blood vessels. .the shockwave of so much power from a shotgun or rifle will explode the head imo with magnum slugs or rifle bullets , the higher velocity and higher energy the more likely Death.
see the soft palate is only about 4 inches away from the brain stem
i would search for 3 inch magnum slugs 12 gauge if using a shotgun