Don't know of any hearing protection for dogs & never worried about it. They're much further away from the gun blast than you are which does make a big difference. I've never known a dog to lose hearing due to being exposed to normal shooting. 'Course, there's bound to be somebody in Boulder, CO who'll come up with "a study."
Too, their hearing is much more acute (which, I guess could be a good case for their hearing protection) - but it never seems to be any problem. & sadly (so very sadly), they don't live nearly as long as we do so the cumulative effect couldn't last all that long.
I worry more about "trunking," etc. dogs where they'll be exposed to carbon monoxides, etc. from car exhaust.
My main concern for my dogs' hearing loss is their having to listen to rock 'n roll at full blast on the way to & coming back.