Removing carbon from face of barrel extension?
I have a new upper (only ~500 rounds through it) and I went to clean it after taking it to the range. I noticed that where the BCG came into contact with it, the face of barrel extension was shiny. All around where it did not contact had a coating of carbon - nothing thick, but enough that it looked parkerized/discolored. I've never had this happen before with so few rounds. I believe this to be carbon - how do I remove it? I tried everything but it's almost like the carbon is just thin enough that it's smooth and scrubbing doesn't do much. I used some KG-1 carbon solvent and it didn't appear to do anything.
Any tips?
http://s9.postimg.org/90p5brthn/photo.jpg