well, we all know that enough electricity can kill and incapcitate. the technology is there to make a stun gun capable of putting out enough amps to incapacitate so i would guess the reason that those stun guns don't work is because they don't want to get sued to wrongful death.
any stun gun with enough power to actually knock someone out would have a great chance of killing someone with a bad heart or just bad luck.
might as well carry a gun, its effects are more predictable and you won't get laughed at by perps who have been zapping themselves with stun guns and know they don't work.
fiddler