Forcing cones are notoriously rougher than the bore. I believe Mossberg chrome lines these bores so be cautious with forcing cone reamers and such. If they have chrome lined over a roogh forcing cone that is exactly what you will get. A chrome lined rough spot. 50/50 acetone / atf should loosen any debris stuck at the forcing cone. Give it a try with a good soaking followed by a bore brush on a drill. Can't hurt.