some m4’s do that, some don’t. Never seen it be more than cosmetic.
10,000 plus rounds. Nothing wrong with my carrier or anything else. Shit happens.
http://www.weaponevolution.com/forum/showthread.php?1744-BOLT-CARRIER-LAPPING-DEBURRING-PROCEDURE
I used to lap new carriers, but found that the only thing I need to do is take a smaller jewelers file and break the edges on the rear of the carrier where the channel starts for the buffer retainer. Rub your finger over the back of the carrier. You will feel the burr. Hit it with a file. then some cold blue and your good.