Had this baby about 6-8 months.  Shelter rescue.  Apparently, I have never taken him shooting before this Sunday.  Not sure how that is possible, but the wife verified that I had never taken him to go shoot.

Shot a single 30 round magazine from my AK.  Moose literally shit himself and ran under the house.  Took about 30 minutes to coax him out.  He spent the rest of the evening Sunday in his crate shivering and whining.  Too him about 45 minutes to even eat his dinner.  And I think he was starving at one point in his life.  This guy has never shied away from his food bowl before.

Heck if I know.   My last dog didn't even like it when I shot a .22 pistol with subs and a can.   Just didn't like it at all.

How to cure a gun-shy dog

My dad's hunting dogs were the opposite. They would go nuts if they heard a loud noise. "Where's the bird, Boss? Where's the bird?"

This. Basically start with quiet reports at distance and gradually bring them in closer/louder as the animal adapts, and create an association with positive events. You can do this with any domestic animal (e.g., shooting from horseback).

Every responsible dog owner should do this whether they hunt/shoot or not. Every time someone whines about fireworks traumatizing their dog I think “Well it’s your fault. You could have prevented this but you chose to be lazy.”

Consider using a suppressor?

Gradual introduction to gun fire.  Helps if you have a helper who will be a distance away and then move closer.  Starting with something softer like a 22 or cap gun, move up as the dog is comfortable by their body language and reaction.  Associating it with something positive works as well such as feeding or play time.  

General rule is you go as fast as the dog is comfortable and back off as necessary.  With my dog I would bang pans together while he was eating as a pup to get him ready for intro to gun fire, worked out fine and we were using live ammo near his hide quickly.

It's hard to get some of them to ever tolerate loud noises. My long gone dog Gus was a basket case every fireworks season from day one after we got him at about 3 months, and I tried the slow buildup playing a CD I had with shotgun and rifle shots getting louder and louder, but it didn't seem to work out. He would basically ignore it until it hit a certain level, not anywhere close to even any kind of actual gunfire, then totally go into a shutdown, crawling under furniture and shaking like he was a Parkinson's patient. This reaction stayed the same for his entire 14+ years. He didn't really need to be scared of anything, the bite force he had alone would stop a lot of dogs in their tracks if he was pushed to the point of anger. For some reason, most pics don't seem to show just how massive that head was.

After he was gone, I got two Doberman/Lab/Husky mixes, King and Molly, and they couldn't have cared less about the neighbors shooting off their fireworks, even at 6 months old. They would sit and watch them! After a while, the noise seemed to annoy them, so they would want to come in. While Molly was a basket case about going to the vet, King was basically fearless, only scared of the vacuum (An identical one at work didn't even make him move after bumping him with it, and my vac at work didn't scare him, only at home, weird.) and a ceramic Cocker Spaniel that he suddenly became aware of one night about 2 years after we got him. It "outstared" him, and he freaked out. Even after he relized it wasn't real, he couldn't make eye contact with it, it was fun to move it around and watch King catch a glimpse of it, and look away. King and Molly at 6 months or so:

It's hard to get some of them to ever tolerate loud noises. My long gone dog Gus was a basket case every fireworks season from day one after we got him at about 3 months, and I tried the slow buildup playing a CD I had with shotgun and rifle shots getting louder and louder, but it didn't seem to work out. He would basically ignore it until it hit a certain level, not anywhere close to even any kind of actual gunfire, then totally go into a shutdown, crawling under furniture and shaking like he was a Parkinson's patient. This reaction stayed the same for his entire 14+ years. He didn't really need to be scared of anything, the bite force he had alone would stop a lot of dogs in their tracks if he was pushed to the point of anger. For some reason, most pics don't seem to show just how massive that head was.

After he was gone, I got two Doberman/Lab/Husky mixes, King and Molly, and they couldn't have cared less about the neighbors shooting off their fireworks, even at 6 months old. They would sit and watch them! After a while, the noise seemed to annoy them, so they would want to come in. While Molly was a basket case about going to the vet, King was basically fearless, only scared of the vacuum (An identical one at work didn't even make him move after bumping him with it, and my vac at work didn't scare him, only at home, weird.) and a ceramic Cocker Spaniel that he suddenly became aware of one night about 2 years after we got him. It "outstared" him, and he freaked out. Even after he relized it wasn't real, he couldn't make eye contact with it, it was fun to move it around and watch King catch a glimpse of it, and look away. King and Molly at 6 months or so:

1. Start the desensitization as early as possible.

2. Begin with low-grade gunfire using a .22 or being far from the firing line. Gradually expose the dog to louder gunfire over weeks to months.  I've always done this at a range with others shooting.

3. Associate gunfire with something positive. I had a GSD who was food driven, so I gave the dog a treat when gunfire is heard.  For those who learned about the "Pavlov" response, this GSD would drool in anticipation of the treat whenever it heard shots fired.  You gradually transition from a treat every gunfire to periodic, random reward when it is heard.  The drooling usually stops when the treats are less frequent. A different GSD was prey driven, so I used ball play as the reward.

"Shot a single 30 round magazine from my AK.  Moose literally shit himself and ran under the house."

Your first choice to introduce an animal that has likely never heard gunfire before, no concept of what a gun is other than "a LOUD BANG machine!" and has better hearing than we do is to light off a magazine from an AK with likely no form of protection?  I don't blame him for shitting himself and running; he likely was in pain and thought you were trying to kill him.  You now have made the hill steeper and the rock you are pushing up it much heavier.

Others here have already posted good advice.  Start as quiet as possible and work up slowly from there.  At this point don't be surprised if you never get there.