I believe an Akita's tolerance to gunfire is genetic. I have trained all 4 of my Akitas the way my family trained our hunting dogs many years ago.
First, 1 person takes the dog off 100 yards or so. Person#2 then fires a 22LR AWAY (180 degreed) from the dog and the reaction observed. Gradually move the dog closer until you get a reaction. Eventually, (hopefully) the dog will be within a few yards and ignoring the sound.
Then take the dog off 100 yards again and repeat the process with a shotgun or rifle.
Keep in mind you dont want to damage the dogs hearing, so dont get too close, especially with the more powerful guns. When you have done all of this, you will know your dog's tolerance of gunfire. Also be sensitive to a pup's reactions. You may want to let the dog mature and get his adult confidence before exposing him to something alien that may frighten him. Know your dog, and dont test him until he's ready.
My personal experiences:
Male #1:
NEVER reacted to ANY gunshot, except he wanted to go and see where the round was hitting. (he saw the dust and heard the thud 100 yards off). Totally oblivious to gunfire.
Female #1:
At the first shot, drug my wife 100 yards and hid under truck. Gunshy. Period. No reason to torture her further.
Male #2:
Doesnt like gunfire, but doesnt shy from it. But would just as soon I didnt shoot.
Female #2:
Not gunshy.Neutral on it. Treats it the same as a car starting. Actually, she doesnt like car rides, and would rather target practice with me as ride with me.
So, Akitas are all over the board when it comes to gunfire. And I think which dog you get is solely determined by their genetics.