People survive gunshot wounds because the bullet does not hit the right spot. With a little bit of research and planning, death by gunshot to the head is practically guaranteed and instant. The key is simply to hit the brainstem, which is done by aiming directly between the eyes back to the base of the head, or by aiming laterally directly on or slightly above one ear, to the other ear. Obviously your aim will not be perfect, but if you use a hollow point bullet, which has a much greater area of internal damage, the brainstem is big enough that the margin of error is very wide. The brainstem controls our most basic survival functions as well as connects the rest of the nervous system to the brain, so if it is damaged, there is absolutely no chance of survival. It'll be like turning off a light switch.
This is how I plan to go, so I've done a lot of research and planning into guaranteeing my success.