How have you been cleaning it after BP use?

It's going to be difficult to find a more effective solvent for BP than hot water. For a bore preservative I never use a petroleum based product. One of the many other treatments are better than petro. I use one of the two I use in my muzzleloaders. After a good rinse with boiling water either Bore Butter or my home made lube of 50/50 beeswax and castor oil or mineral oil or bear fat or deer tallow or some other natural oil/fat.

NRA Benefactor 2008
NRA Patron 2007
NRA Endowment 1996
NRA Life 1988