There are laser bore sighters that attach magnetically to the face of the muzzle. They work on any size bore or cartridge since they don't require anything inside the chamber or bore. There are various opinions about how well they work; many folks seem to have little success with them. Personally, I love 'em. They've worked great on almost every rifle I have tried them on, usually much better than any actual boresighting done by peering down the bore with your eyeball.
The catch is that they require the muzzle face to be 100% perfectly perpendicular to the bore, and you won't be certain this is the case until you use it to actually boresight the particular rifle. So no guarantees in this case, but odds are that if it gives you a good match of laser dot to sights you will probably be okay.