Here's a pic of a Colt 10.5" SMG barrel. Never fired.
If what you are seeing is a discoloration (stain) in the chrome or the step from the chamber reamer and the chrome flashing past the edge, I think you are worrying about nothing. If you see peeling, then you have a issue.
Chrome plating is performed in a very caustic environment, acid and electrical current. The acid is dark and can leave dark film marks (stains) sometimes if not washed enough. Oiling chrome after plating is common and that oil can dry and stain. No big deal. In chrome plating, the chrome builds up faster on edges rather than surfaces. Plating thickness of only .002" can result in large burrs growing / protruding from the edges. These edges have to be ground or polished away. Maybe you see de burring or polishing marks?
The gun runs? If so, Id rather worry about the rifling chrome rather than the chamber..... The barrel should last you and your kids lifetimes........
Do you have a close up pic?
[img]http://users.zoominternet.net/~picplace/9mmchrome.JPG[/img]