Mark's real-real close with residue theory... actually he's 99% right. The 1% he's missing in his post is what turns the fouling white. That is salt concentration from your powder.... usually occurs after the bore has not been cleaned for a spell or been left out in the open air of your home, garage, auto... etc -- not hardly related to plastic fouling there. Best to keep your fouled MLs in a hard shell, poly-foam gun case that keeps it air-tight inside your home after range work. When hunting with a fouled bore, you can do the same but the rifle temperature must be acclimated from cold to warm much more slowly.

Remember what I mentioned about cleaners.. use the proper ones & swab frequently for best accuracy & easier removal of bore gunk.