I had a beagle that was gun shy so bad she would crawl up into the fender wells of my truck to hide. Broke her by taking her to a trap range in the country, stopping about 1/2 mile away. I brought her favorite toys and lots of snacks. At that distance she was a little jumpy, but with all the snacks and so on, she got comfortable with it. The next week when the trap shoot went on, I moved her up, and so on until I had her walking behind the firing line without any problem. ;D
The britt I currently hunt with was broken to the gun the same way. It was a charity trap shoot for the local volunteer fire dept, and by the end of the day, all the kids had him on his leash walking and playing with him.