I would use the younger horse, he's already proven to be calmer around the guns. To get him used to it, leave him in a field close enough to the shooting range so that he can hear and see everything that is going on. Make sure he has a friend horse with him so that he doesnt have any reason to get aggitated. After he is completely used to you shooting from some distance away, cut that distance in half. Eventually get to the point where your standing beside him shooting. DO NOT tie him while getting him used to guns, horses are flight animals, and if he realizes he cannot run from this he will panic and it will only set you back in his training. Once he is perfectly fine with you shooting right beside him, get on him with the gun and swing it around to get him used to having the gun around his face and above him. From there start taking about one shoot off him a day, and praise him like crazy afterwards, then stop. Don't make him do it too many times in a day, or he'll start to resent it. I hope this helps!
For without a horse, i could not fly.