Most dog attacks aren't from "properly trained dogs". My Judo instructor (Retired Marine Sargent) is who taught this. He claimed to have used it on a big old shepherd mix in Vietnam. He said it jumped up on him. He blocked at the throat with his right forearm/elbow and was able to grab a paw as he pushed back and unbalanced the dog...hung on to the paw and came down hard with his left and snapped the paw...and the dog wanted nothing more but to get away after that. It's not like we had the opportunity to try it out on real dogs so this was all talk. Just like, you can't actually train a dog to fight through fatal gunshots. I'm sure every scenario is unique and nothing is 100% either way...but it's better than just letting one rip your throat out. Ya gotta try something besides screaming and flailing.
Yes, but it's best to try things that are actually likely to work.
Also, you may not be able to actually train a dog to keep coming after taking a gunshot, but you can sure as hell train them to keep coming through gunfire and harsh "physical input".
