What I would do is what you are doing: agonize over the decision for a while and then make it, somehow. If your father has or can be persuaded to get the appropriate license, perhaps you could make that revolver the gift instead of the shotgun, with the provisio that he shouldn't feel he can't sell it to get a shotgun, you just wanted to give him the option.
If that isn't feasible, take him out shooting with that revolver, telling him you've decided to sell it so he ought to come and experience it with you. Don't tell him why. Then when you present him with the new shotgun, tell him it's why you sold that revolver. He's your father, you can safely assume you have made him cry before, but this time it will be in a good way.