I start my conditioning to gunfire process by clapping my hands together each time a pup goes to eat. i start a distance away with fairly soft claps, and progressively get closer and louder with each meal, all the while observing for any signs of aversion. No problems noted, I progress to louder and more frequent and random claps to simulate multiple guns firing. Eventually, I take the process outside and switch over to a 209 primer revolver and use distance to control the volume experienced by the pup. I like to have the pup engaged in play with another dog and continue to monitor for any negative reactions. Next phase I move on to sub gauge shotguns and progress to louder loads while closing distance over days.
Me own opinion on this is that so much of it is in the control of the trainer, why leave anything to chance? And I have never ruined a dog on gunfire introduction.
Irishwhistler