Next to having a dog killed by a passing car, there is probably no worse feeling for a bird dog owner than having the hunting dog he paid dearly for, trained, raised carefully and conditioned to loud noises, come crawling into your vest at the first gun shot on opening morning. If this sounds like a personal story, it is. It was devastating and never resolved.
Craig Doherty suggests most gun-shyness problems can be resolved, but the owner may have to go back a long ways in the training process.
Source: The Pointing Dog Journal, September/October 2016, page 36. (direct link unavailable)
Once the dog is having fun running and chasing birds, that’s the time to reintroduce the gun.