I don't believe its plastic after an hour of scrubbing. I think it may be rust built up and now is severly pitted. Besides, I have never heard of plastic build up like that in any shotgun. Take that brush or get a new one, take an old shotgun cleaning rod, cut off about 6" long, attach the brush and put in a cordless drill. Try cleaning out the chamber area that holds the brass of the case. I have found that more times than not, thats what causes shotgun shells to hang up in a situation like this. Does it do it with all shells or just high brass/magnum shells?