“I currently civilize feral cat, for fun”
Well, you certainly have a dangerous side to you. LOL. I am guessing that the feral cat training doesn’t involve shooting a gun over their head. Seriously, my next dog... I am doing your training technique. You can’t explain that the booms are simply noise and some of the larger dogs can get into trouble. Our vet reported that one GSD got so scared that he jumped through a set of windows. Lots of stitches and treatment for dehydration when they found him miles away several days later.
A lot of gun-shy Labs wind up in rescue or dumped because people tried the sudden introduction method and it failed. We've seen the results at the hunting club, it's not pretty.
I have three hunting retrievers who LOVE loud reports, the bigger the better (might mean bigger and better ducks, right?) I used the gradual introduction method, and so does everybody in our club. If your dog's a retriever, throw bumpers, if not, feed the dog while a friend fires a cap pistol a good distance away. If the dog doesn't flinch or startle, have them move closer gradually. Ignore the noise, don't even mention it to your dog, just keep feeding or throwing. The idea is to associate the noise with FUN. Step it up to a blank pistol (I use an old .38 S&W with primed empty shells), then to shotgun primers, then to AKC "poppers".
My dogs are all staunch to any kind of report, including fireworks, thunder, and car backfires. They're also de-bolted and force fetched.