Line in my barrel

[Fed168]

I noticed a black line in my barrel (stainless steel Shaw, AR 15) about where the gas ports should be. The line doesn't look like ports, and is perpendicular to the rifling.

I was thinking that some of the coating came off the Dewey, but after trying to clean it, it is still there. I tried shooting, and the line is still there.
No malfunctions, but I haven't put it on paper yet.

Any thoughts?