Well I thought that the bolt/stunner were obviously meant to stun and not to kill. Clearly a lot more misinformation than I thought out there.
I qualified as a vet and what they teach us and what we see in various abattoirs is this: animals arrive, vet is supposed to check that each is healthy enough to be killed, they are only supposed to stand in waiting pens for no longer than 2 hours (30min if birds/rabbits), if longer they must be given food and water, if staying overnight they must have shelter from the elements.
When their time comes, they need to go in single file towards the stunning area, where they are stunned using various techniques. The following animal is not supposed to see what happens to the one in front, although I will agree that they are not stupid and even if they don't see, they can still hear and smell death.
Bolts are mostly used for cattle, horses and sheep. For pigs it is easier to use electrocution, or some big places have CO2 chambers. All of these techniques are meant to make the animal unconscious, and not to kill. If they are killed during stunning procedures, the staff hasn't been properly trained. Being unconscious does not mean that the animal might recover. In fact, most stunning techniques were devised to induce permanent unconsciousness.
It is duty of the staff to hoist them asap and bleed them by severing he big vessels in the neck. THAT is when they die, from terminal bleeding and not before.
So the stunning is supposed to ease their passing from alive to unconscious to dead without feeling too much pain.
If somebody said that ANY killing method is better than another because there's no pain, I would call that b*****t; in any method there is a certain amount of pain, and stress due to the handling (I agree, most rough in some cases). It is our duty to make the amount of pain and stress the smallest possible, but it cannot ever be completely annulled.
So I think that whichever way it is done, whether Western or Halal or Kosher, if done properly and following the rules none is supposed to be better or worse for the animals. Even those methods that were based on ancient religious practices still keep the animal welfare in high regard. They all strive to make the process as quick and painless as possible - repeat, if done following the rules.