"Is that just an old wives tale? I must admit I have never worked with horses so I am quoting only what I have "heard"."
Absolutely not an old wives tale, a bullet into the forehead has always been an acceptable form of euthanasia. It's quick and done correctly, relatively painless. I've seen horses put to sleep and it was horrifying. Some may drift peacefully off to sleep but the ones I've witnessed fought death and it was heartbreaking.
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.