I don't think you can use a sound system inside where the dog is completely comfortable to train for sound sensitivity.  I know my dogs never cower when there is a loud noise when they are in the house, they always want to go investigate.  Now outside, that is a different story.  You definitely should not baby them and you should ignore it, slowly getting them further away from the sound and distracting them.  I know starting my first GSP out, she was afraid of the gunshot.  Would tuck her tail and wanted to get away from it.  Of course, she had never heard a gunshot before either.  Now, once we got her bumper out to retrieve (we were working on the water retrieve test for the GSP nationals) she totally forgot about it.  We did this, slowly getting the gunshot closer, but always while she was doing her retrieve from the water.  This definitely worked and at the Nationals, we were successful in getting her NRD title on retrieving a quail from the water.

MelB - we have a very loud teeter we train with.  While it is the sound they notice first, it is also the big movement of a full size teeter that is also the issue.  We start out with a baby teeter, not having them go across it, but we put Ceaser (the dog food in the small containers, we call it crack for dogs) and have them get it from the end without them having to make it move, graduating to moving the food further away on the teeter to have them step on the end of the teeter to make it move.  Once they are very comfortable with that, we will then have them go across it.  You don't want train them to hesitate at the pivot point, so don't have them go across a baby teeter until they are comfortable with stepping on the end making it move and the sound of it.

Ok, here's a question...maybe I just missed it. If you get a really good response from a distance like lets say at 200' your dog has no idea a shot even went off, is it ok to move closer during that session? And if so how close is too close in one training session? I'm just getting ready to start Dax with gunshot desensitization. As a younger pup I would pop bags in the house, clap my hands, and knock on his metal bowl really loud when it was time to eat. He generally seems to ignore loud noises including fireworks, thunder, and shooting at the gun range over the hill from us.

My opinion is that we are not looking to see a reaction. In other words - we don't continue until there is a reaction and then stop or work thru it. What a dog does with distant noise can be a far cry from what they do at close range noise. Once you see that startled reaction from a pop,which is may not be a bad thing, the next pop the dog should see a moving object so it can follow it with its eyes or run if it wants to. When we do clay shooting the dog are watching the clays more than the shooter. The movement brings out prey.

Sensitivity (nerve) is inherted but much of what a dog is is a result of exposure and training. We can never know what a dog can or can not be or what their strengths and weaknesses are until we expose and train.

My general opinion is that once  (most dogs)a dog comes to the realization that a shot produces a bird/retrieve for them, they are anticipating the noise.There is that connection. I don't do specific gun intros. We shoot a lot and the dogs learn what it means thru association. Then there are those dogs who just aren't cut out to be gundogs.

 Vizslas are Gun Dogs/Pointing Breeds too and run in the same hunt tests, field trials, etc as GSPs.  They were breed to be versatile dogs much like the GSPs, just from a different country.  In fact, my boy, Ringo, has been braced with Vizslas several times at Hunt Tests.  Vizslas are very nice dogs and if I wasn't already sold on GSPs I might have Vizslas.  If  you are thinking that you might be able to train together, absolutely.  Other pointing breeds that you will see in hunt tests, field trials, etc are the setters (Irish, English, Gordon), Griffons, Spinones, Vizslas, Brittainys, etc.  Here is a link to a list of pointing breeds in Wikipedia.  Note that not all of the these breeds are recognized by the AKC.  However, they are all pointing breeds and are used to hunt upland birds and in some countries game.  Note that these breeds may have different hunting styles and you will certainly find differing opinions about how these breeds hunt, but they are all Gun Dogs.

I couldn't have described it better! This is exactly what Casey has learned: gun shot = bird. It seems like association is how he figures out everything and anything.