The bird with noise method works well especially if the dog isn't already gun shy. I think at this point you should seek help from someone who has experience undoing gunshyness. The last thing you want is for the dog to fear birds by shooting a gun while the dog is in close contact to live birds. This method works well if done right but has also caused a timid dog to fear birds as well when it starts hearing gunfire which it already is afraid of. There is no sure fix and what does work for most may or may not be helpful to you. You need to take 3 steps back and not go right back into gunfire even at a distance for now. Take your time and allow your dog to adjust to progressively louder noises increasing the volume SLOWLY and not in a hunting scenario. Meanwhile build your dogs bird drive with penned birds in the yard, teaching him birds are the best thing ever so that he doesn't later confuse birds with a negative such as gunfire which it currently perceives as a threat or danger. Once the dog is more confident and not skittish to loud noises then begin combining the two. Be Patient and Persistent.