BP will melt most plastic shotshell cases and do the same to the plastic wads. Some it will burn and char them more than melt them though.
It will deposit the plastic onto the bore surfaces. If you shoot a lot of them the plastic build up can be considerable with some brands of hulls and/or wads.
Strings of melted plastic in some instances. Other times just an ultra thin coating that gets broken up during the scrub & wash cycle of BP cleaning.
That might be what you are seeing.