Probably a more complex problem would be the action itself. Some of these, Encore, H&R and some others, are designed to deal with cartridge pressures and strains. For the others, if you wanted to put a cartridge in the barrel and screw in a breech plug before firing, it might possibly done with some modifications mentioned above. If you would want to design a cartidge type rifle on one of the more typical less expensive actions, you would need to add a locking mechanism, something like underlugs and a cross bolt. you would probably have to put in a different hinge pin as well. Even black powder cartridges put very different strains on the action than a sealed breech muzzleloader. I really don't think you can do it without an almost ground up build. Not a gunsmith and don't pretend to be, just my opinion.
Bfly
