Stop testing the dog. Its sounding like you are making him gunshy. It could be that he's running to you for protection and looking around for what made the sound. A gundog that is not gunshy will ignore the sound; a gun conditioned dog will look around for a bird in the air immediately after the sound. Personally, I would spend as much time as needed for gun introduction b/c gunshy is harder to cure.
Pick the DVD mentioned by TakemDrake (I've also heard alot of good things about it) and look at it before you do anything.