Doh!! I looked down all four of my 870 barrels, they all have forcing cones
The forcing cone is the part of the barrel that is always the dirtiest and most difficult to get clean. If you look down the barrel from the breech end while holding it up to a light source the forcing cone is the dark area immediately forward of the chamber. It appears to be a dark band because of all the plastic fouling that builds up there
Poor Remington, what IS happening to them?