David Canterbury of Wilderness Outfitters has a
video on yoofloob pouring round balls and loading
an H&R break open 12 ga as a muzzleloader and
several others along the same lines with procedures
etc. etc.
Would guess there would be some definite do's and don'ts associated with that because most shotguns have a constriction (choke) at the end of the barrel that a muzzleloader rifle doesn't have. Loading appropriate slugs in shotgun shells with some type of wad/patch over blackpowder might be a better solution than using a shotgun as a muzzleloader, at least until your shotgun shells were no longer usable. If shotgun shells aren't available, you're probably talking massive nuclear war fallout scenario with most of the U.S. population gone. Obviously Alaska has different supply chain concerns than the Lower 48, though.
