IME as a trainer of hunting dog owners, breeder and hunter, gunshyness is always a man-made problem, the product of an ignorant owner and a sound-sensitive dog. Too many folks rush the pup to the shooting range "to see if he's gunshy", obviously going for the gold in the Idiot Olympics.
As a breeder, I check for sound-sensitivity as I raise the pups, so I can deal with it early on. As a trainer, I don't even think
about introducing the gun until the pup is totally focussed on the mark and retrieving at 100 yards, at that point they barely even notice the shot. From then on I pretty much use the method described in the sticky, but I do not use birds (have never found it necessary) until I am sure the dog does not have a problem with the gun that he may associate with birds. I have never had a gunshyness problem this way. I have seen dogs with such a severe problem that they would leave the county at the mere sight
of a long gun, and God forbid you rack a shotgun within hearing range. Best to avoid creating the problem in the first place, very difficult to get a dog by it.