Pups go through stages of development when they're more likely to have a fear of something ingrained in them. If you give them a shock it can make them shy of whatever it was. It can happen to any pup. This is why I pretty much let a pup be a pup the first nine months. I make sure there is no gunfire near the pup the first nine months.
If yours was older than that at the time, and it sounds like it was, then you've just got a cull through no fault of your own. They don't all work out. To make a breed culling must be done and continued as necessary. Considering what breeders charge for bird pups, they ought to provide another. That's not how it works, usually, which is why 99% of bird breed pups are grossly overpriced and there are plenty of people with useless furbabies at home.
Now, practically speaking, nobody ever culls them anymore. That's how you got a dog that was not genetically predisposed to love the report of a shotgun. The least you can do is cull the reproductive system. That leaves you trying force the dog it to do a job it doesn't want to do. There are lots of methods and they rarely work. What it comes down to is whether the dog wants the bird enough to tolerate the gunfire. If you can get it addicted to having feathers in its mouth it may tolerate the painful scary noise.
Good luck and please don't let a cull reproduce. And don't be shy letting people know where the cull came from if they won't make it right.
