A lot of smokepole people have been getting rude surprises after switching to plastic sabots. The sabot has been leaving a plastic residue. The residue is promoting rust.
The best thing I’ve found for plastic residue removal is Ed’s Red. It’s basically equal parts kerosene, automatic transmission fluid, acetone, and mineral spirits. This works good on just about any barrel, but it is especially good on shotgun and muzzleloader barrels with plastic residue.
The recipe is everywhere on the web, I think the best page for discussion of Ed’s Red is Fr. Frog’s web pages. If you’re interested, and can’t find the recipe, let me know.
After normal cleaning, swab Ed down the barrel. Let it sit a week, and then do it again. If you have build-up, you’ll be surprised what comes out.
If you’re not using plastic, I use this recipe as a black powder bore cleaner:
One part rubbing alcohol, one part hydrogen peroxide (the off-the-shelf product, mostly water) and one part Murphy’s oil soap.
On black powder and pyrodex, it seemed to do a better job than any other in loosening stuff. After I switched to this mix, my rusting went away.