Monk and Dan are most likely spot-on, as micro pitting will appear as a dull surface, yet not present as a constricted area, as a deposit would provide. If there was a deposit of plastic or anything else, there would be a constriction 'felt' in that area.
Since there is no constriction & chlorate powder was used, ie: Pyrodex - it is most likely chlorate pitting as the two gentlemen suggested - in a minor state, perhaps - at this time.
Lapping is what I would do, to alleviate the 'problem'. Been there, done that.