Originally Posted by
falcon
Been shooting inline guns since 1999, sometimes with large powder charges. Have never had any plastic fouling.
Maybe you just can't see it. It leaves only a very thin film in the corners of the rifling. And it adheres strongly to the bore. A bronze brush may or may not remove all of it. Even shotgun shooters use a plastic solvent to remove this film from their barrels.
It is a possibility but I now tend to agree with the other posters on the enlarged flame channel.