got this of another site hope it helps
One of the hardest problems to correct is gun shyness or gun sensitivity once it is instilled in a dog. Unfortunately, introducing a dog to gun fire is the one place most people make mistakes. Like all other aspects of field work the introduction of the gun needs to be introduces in steps, with positive rewards and praise. There are varying degrees of gun shyness, from the dog that flinches or runs from the sound of the gun, to the dog that hides behind the furniture at the mere site of a gun. The common thread to any of these behaviours is that the dog