I think if you locked me in a small room with some simple tools (lathe at least) and I HAD to do it before you let me out I'd find a way . But I'd work on a way to expand the lap and not have to keep recasting in the bore over and over again. or make an expanding lap from brass, bronze, cast iron or aluminum. It would still take a long time...might be that a guy could make a guide bushing for both the muzzle and chamber end that ended up lapped to a rifling form when you were all done.

The abrasive will load into the softer material when you try to lap between two things, and one MIGHT be able to cast a lead lap, then use it in turn to make a female lap, and that in turn to make a male lap from a harder material. Lead/tin or cerrosafe for the cast in bore lap, then lap a female aluminum lap (that was split to work like a collet)...then in turn make an expanding cast iron lap. The small size all makes it tougher though .

Still lock a man into small room with a lathe and some hand tools, and give him only water and no food, and he does not get out til it is done .

Bill