Im surprised guillotine isn't on the list!
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Utah has it on the books, and I think they've used it. I think they still have it on the books as an option (I think the guest of dishonor gets his choice of exit method). As I recall, they even had a self-execution firing squad; the person to be executed sits on a chair where the rifles are zeroed, and the pressure of sitting, via a series of levers and/or cables and pulleys, fires the fatal shot(s).
Edit: According to Wikipedia, Oklahoma has firing squad as a "secondary method of execution," and is the only remaining state that allows firing squads. Utah apparently outlawed death by firing squad in '04, but it was not retroactive, leaving four death row inmates who could opt for it, and one of them did in 2010.
Interestingly to note, death is by blood loss to the brain. There were reports back in the day that said the heads would blink for a second or two before expiring as there was enough 02 left to keep them alive for moments after getting their head lopped off.
- serial killers and most murders, forcible rape, an individual with a large number of violent felonies, etc. - the method should be as painless as possible to achieve the desired result....otherwise it becomes vengence instead of a simple removal...my 2c