I've had one horse euthed in the last year, and was with my friend when she had 2 horses euthed this past year. I've been there when 3 other horses were euthed over the past several years. Each time it went well, if killing a horse can ever go well. Lots of tears in each case, but the euthanasia was peaceful; horse was heavily sedated, actually "out" and then the other chemical was introduced and in each case, within about 1 minute, the heart stopped.

I've heard that it can go wrong, but have never personally witnessed that nor have I ever known anyone IRL that has had it go wrong.

I don't know about shooting, I'd think if you know what you're doing that it would be pretty damn quick and efficient. However, a guy I know tried that shooting behind the ear thing. Yeah, the horse trotted away after he shot it. So then he had to chase it down and shoot it again. A week later when he was telling us about it he was still completely freaked out by it, and it just gives me the shivers even thinking about it now.