At this point I would like you to become somewhat more intimately acquainted with the actual process of raising livestock and slaughtering them. The piston guns aren't perfect. They really aren't. The electric cages aren't perfect. They really aren't. Most slaughter is going to involve both the stun and the bleed-out. Depending on how you are stunning the animal isn't always unconscious, it's just sometime too incapacitated to move enough to hinder the process of killing it. Sometimes the stunning wears off before the animal has bled out. Aside from that, much of the trauma that comes from the entire process of slaughtering something does not come from the actual slaughter itself, but from(as El Mac points out) the process leading up to it. This is where the best process has been made. Slaughterhouses that use stunning devices and those that don't can both use Grandin-esque new techniques, especially considering at the end of the line the animals have been cut and hung whether or not a stun gun/cage/shock was used or not.
More interestingly is that you put enough regulations on this sort of thing and the only plants you'll have left are the huge industrial ones that are able to comply. Those very same plants that tend to be the worst at minimizing immediate-slaughter fear and mistakes. Ol' Bubba in the back 40 that probably does an overall pretty good job simply can't do it legally anymore. Two steps backwards for great personal feelings of progress! Virtue of said feelings not relevant.