My understanding is that the destruction of the brainstem results in instantaneous death.
I stumbled upon a crime scene analysis the other day (can't remember the URL) where the victim appeared to have CTB with a shotgun. He was lying on his side, partly atop the shotgun, chair overturned on top of him as though he'd shot himself and tumbled sideways. He had used a 12ga with #6 shot --which, interestingly, had not blown his head apart; no exit wound, but the shot had utterly destroyed his brain, with the pellets remained contained within his skull. (The x-ray was quite something to see.)
However, the vic presented with both Battle's Sign (bruising at the ears) and "raccoon eyes" (black eyes), and his arms and wrists were tightly curled. All of those are signs of severe brain trauma, but the ME's report concluded definitively that the vic did not die from the shotgun shot, because 1) the heart needed to have been functioning for those particular types of bruising to occur, and 2) the curled arms and wrists could not have happened unless the brainstem had been intact to send nervous impulses to the arms. In other words, the Battle's Signs, raccoon eyes, and curled arms/wrists proved death was not instantaneous, but the shotgun blast would have resulted in an instantaneous death that precluded those signs, QED the shotgun was not the cause of death. After looking more closely at the vic, it turns out someone hit the vic on the head, delivering fatal --but not instantaneously lethal-- blunt force, and then shot the vic in the mouth with a shotgun to make it look like a suicide.
But the ME was unequivocal: that shotgun blast to the head would have been instantaneously fatal. Life support and brain activity would have ceased in a fraction of a second.
So if you do it right, yes, it's instantaneous.
As I said: aim is everything.
