About a year or so ago I saw a video of a guy replacing the primer in a spent shot shell and then using his single shot break over shotgun as a muzzleloader. At the time I thought well, OK, that's not very exciting. But later I thought about those rifled slug barrels and muzzleloading boolits. I don't have a break over shotgun, much less a rifled barrel slug gun, but surely there's somebody out there that does and maybe someone that has tried out muzzleloading with one. Any way, it's just something that I got interesting in looking at and thought I mention.
A 20Ga with enough black powder in it to get the bullet (maybe a Lyman mold?) up past the chamber, now that might be interesting to try out.
Had a long barreled Stevens side by side 12 years ago. Kinda wish I'd thought of trying it out with patched round ball.