Truthfully - a shotgun only shoots 50 - 100 yards with punkin balls and the optic's for a rifle scope is not designed for a parallax view of that distance.

You should buy a scope for either a shotgun or designed for a muzzle loader. I have personally witnessed several scopes that gave up the ghost after only a couple of shots - being mounted on a shotgun.

A couple of them, you could shake them like that shaker exerciser you see on television. There are some manufacturers that will refuse to honor the warranty if you do not use the proper scope on the proper gun and they find out about it.l

I think that the secret of accuracy would be for him to have the barrel pinned permanently to the action. But that is just my opinion.