I don't know that I'd jump to the issue being insufficient clearance. Some marks on the barrel chamber area and lugs are common. A gun doesn't run 5-6K rounds and all of a sudden have clearance issues. My guess is something got in the action and created that burr. I'd get rid of the burr before shooting it anymore, but wouldn't be too worried once it was gone. Shoot it some more and keep an eye on it.