If I am interpreting the photo correctly the contact is on the "outside" of the lug farthest from the firing pin?
That would correspond to the deepest part of the recess. You might make a dental type pick out of a soft nail and try to scrape more gunk out of there. A nail should do no damage to the semi case hardened receiver, if nothing else you would be able to tell if the base metal has a high spot down there. If it did it would be hard to explain, unless this rifle has seen a huge overload in its life.