I BD always used a cap pistol or a 22 blank or a primed pistol case too.... unlike you though I did it when the puppy was just fed and eating.
Start further away and over a few days/weeks get closer and closer. Be in a position where you or someone with you can watch the dogs reaction.
Ultimately I was able to stand next to the pup and food and shoot the noise maker!!
Don't know how it might or might not work on an older dog. Give it a try; dogs like to eat ... lol
When I was in my teens, and....dinosaurs walked the earth, I used to rabbit hunt with beagle. Sometimes we would get a dog that simply was "gun shy" and there was not turning that dog around no matter what method we used to acclimate the dog to a shotgun going off. Unfortunate to say, but....some dogs just do not like load noises.