Most "immortal" creatures are only so until you kill them (i.e., injure them in such a way they can't heal). This deals with one of those, a vampire to be specific. So, I got the idea of inflicting a horrible brain injury with a gun, to assassinate one of my characters. The idea is that the murderer would sneak up behind him stealthily, while the vampire was off-guard, place the gun at the top of his neck or somewhere, and kill him with a single shot.
I searched for "fatal brain injury gun", "spinal injury fatal", "fatal brain injury", "brain stem damage", and "brain stem damage gun".
I speculate that a brain stem injury would be most fatal, as that controls the most basic functions required for life. But I also know that if the gun is aimed wrong, it might just paralyze him and not kill him. The murderer is a trained government agent, so I assume he'd know just where to shoot.
So... there two basic things I need to know.
1) If shooting from behind, at point blank range, what precise locations can someone shoot to kill in one shot?
2) In the 1920's, what guns would I have to choose from? I'm thinking for a stealthy killing, one would use a .22 or .48 pistol. A tommy gun blasting off the back of my guy's head would also be acceptable, but I'm not sure how much force those fired with or if they'd take a longer time to shoot than a pistol, thus making them less stealthy.