I agree with all who said it is dirty and needs cleaning. The same happened to me when I shot slugs from my chrome lined barrel. Being new to shotguns, I feared I had permanently damaged it with excessive slug shooting (I now believe there is no such thing). Bronze brush, a quantity of G96, a lot of elbow grease any quite a few patches brought the barrel back to a mirror finish. Had I known a drill could be used safely before, it would have saved me lots of scrubbing! Drill usage recommendation duly noted.