Henry 1;
Sounds like your going to have to start slow, and at the very beginning. Being a stray you don't know what the dogs experiences have been with gun shots!! He may have been tramatized but good even if not hit.Get the dog interested in something else, food, play, fetching whatever and have someone slap two blocks of 2x4 together, fairly gently to start.A couple of 6 inch blocks can make a pretty sharp crack. Once he stops flinching when the blocks pop, pop them good when you throw a dummy for him to retrieve. See if you can make him associate a sharp crack with something fun. Go slow and good luck. Let us know how it turns out.