Good question Lee. I've never noticed a plastic build up in any of my barrels. After Grouse's earlier post I double checked the bore of my .45 LRH barrel because I recently shot a lot of sabots through it with some pretty hot loads. I even tried a tight patch very wet with carburetor cleaner and got nothing out of the barrel, though I never use a brush in my ML barrels. I wonder if a brush would find some plastic?