When the extractor contacts the lip of the brass casing when the bolt rotates to lock, the extractor slips over the lip of the brass casing, which will cause small pieces of brass to come off the lip of the casing.  If you look inside the gun, you will find small brass pieces on the bolt face, bolt catch area, barrel extension lugs and chamber, and often times in the trigger area.  Generally they are nothing to worry about as this is normal, and simply just clean these out when cleaning the rifle.

CY6
Greg Sullivan "Sully"
SLR15 Rifles
DefensiveEdge.com
(763) 712-0123

What caliber is the AR ?

Sometimes a 300BK can have incomplete powder burn, and that unburnt powder can be confused for brass shavings.

Any Pics of the issue and the headstamp end of the fired brass ?

Easy fix, and under stand that when the bolt goes to  rotate unlock, spent case stays fixed to chamber, extractor rotate on the stationary spent case, and it not until the bolt is unlocked to pull the spent case out of the chamber, that the spent case now start to move.

So with extractor in hand, and small jewelers file.  Do not touch the center of the claw edge, just the sides of any burs.
Need to remove the end burs from the extractor sides, where if burs are at, then going to cause the massive brass shaving as it rotating on the fixed position spent case.


Next, use the file to remove the burs from the side of the extractor rim relieve channel, so as the extractor slips around on the rim at unlock as well, burs here not shaving the rim edges  as well.



A little cold blueing, and extractor will look right as rain,


On the bolt face, take a look at the ejector channel, as well as the edge of the extractor channel on the bolt, since these areas will also being slipping on the back of the rim during bolt rotation with spent case still in chamber, and if either has high burs, remove them a well.



Or, just don't worry about the problem, and in a few thousand rounds of live fire, any burs will be worn off by the softer brass over time.

Also, this includes are burs on the barrel gas port passage in the bore, since with burs not knocked off with a bore brush during rifle cleaning before every put into use, bore gas port burs these will shave brass off the bullets as they pass the gas port as well.