The destruction of brain tissue during the experience of death, while bringing about a swift end to detectable life, may not be a pleasant way to experience death.
1) How much damage a brain can take before it's incapable of experiencing.
2) How fast a brain can have an experience.
3) How long it would take for a supersonic shotgun slug to reach point 1.
I'm extremely skeptical that point 2 is fast enough to meet the deadline imposed by point 3. I suppose I can't say it's impossible, but it seems very unlikely.
For that matter though, let's suppose point 2 is fast enough to meet the deadline. We don't actually know if that is a more unpleasant death experience than being hanged or decapitated. We can speculate about which experience is the worst, but none of the dead can actually report back to us with real data. My personal suspicion is that decapitation is the most unpleasant, since there is a strong possibility you will be conscious long enough to experience your situation, and that experience will be truly unpleasant. But who knows.
I don't think it's reasonable to make the assumptions you make in your three-part calculus, though I do acknowledge that it's a thoughtful way of considering the matter.
But what if an intact brain is an important part of navigating the first 10 minutes or so of the experience of being dead?
I prefer not to have my brain mushed at all, and let it very slowly decompose. Certainly the first hours of being dead, while it's still possible that electrical or chemical activity in the brain is ongoing, I prefer for it to be intact and at peace.
My calculus is predicated on the assumption of a materialistic mind and no afterlife (e.g. there is no experience of being dead, only experiences of dying.) If those assumptions are wrong, then I think there is no firm ground on which to speculate at all (maybe Valhalla is the true afterlife and a gory death in battle is the most form of death the dead find most satisfactory. I don't believe that, but I disbelieve it only as hard as I disbelieve the Christian afterlife.)
In my model of the world, perhaps a brain slowly ramping down is a pleasant experience. That's totally conceivable to me. However the brain being instantly disassembled faster than it can have experiences would be the absence of experience, be it a pleasant or unpleasant.
