I could never shoot a horse tuffy. I was present when my friend's horse was put down with drugs. I'm glad I was, because it went as easy as any sedative.
I could personally never do it, either. But the horses I've seen put to sleep were so bad that I'm thinking nothing could be much worse. POW, it's all over. I had an old sick cow put to sleep once and the vet couldn't get the jugular so he had to do a cut down. A bullet in the brain would have been a million times easier.