Ignoring Carbon Build-up - per Armalite
There are frequent posts asking about cleaning carbon from the AR. We've decided to post our answer as a separate topic for your reading amusement:

Hold off on removing the carbon. Leave it.
Now that we've got your attention, we'll explain.

Deep inside the AR-10 or AR-15/M-16 bolt carrier is a groove at the end of the chromed cylinder that the gas rings slide in. That groove is GRIND RELIEF. It's a production artifact. It prevents a ledge from being left where the grinding for that cylinder ends.

Carbon can build up in the groove and the rear surface of the carrier. It is harmless. The high pressure/temperature operating gas keeps it from building up too much. About the time it gets too bulky, it gets blown out.

Don't confuse instincts for cleanliness that you learned in the military with common sense. It's easy for an inspector to know when there isn't a speck of dirt. It's harder to know what's important.

Any scraper that'll get to it can damage the ground surface inside the carrier. Bad deal.

Clean out what you can with patches or Q-tips, but white-glove cleaning damages more guns than all the shooting we do.