Fifteen years ago I bought a Labrador dog of 18 months. From very good working stock, loved retrieving swimming etc. but soon as the gun was produced, would slink away and hide. I tried all the usual advice but it got so bad that one evening on the Ouse Washes, he left me and went half a mile back to the truck.
Things were not looking good and I thought that I would have to give him away as a pet. I was talking with my wife who applied her long knowledge of looking after a succession of Labradors. We were in the kitchen and the dog way lying by my chair.
My wife told me to get the shotgun and lay it on the floor, which I did. She then got a chocolate covered biscuit, broke it into small pieces and placed it on the shotgun, even amongst the double triggers.
After a few minutes the dog came out from his hiding place and started to eat the pieces. His desire for the treat had overcome his fear of the shotgun.
My wife did this for several days and it ended up with the dog salivating as soon as the shotgun was produced. No more fear of the gun itself.
Further gentle reintroduction to the sound of firing, together with pieces of broken biscuit in my coat pocket, eventually produced an excellent shooting dog, which I kept until the end of his life at 12 yrs of age. And yes, I know chocky biccies are bad for dogs.