Any chance you could post a few pics of the inside of the receiver where the brass is accumulating, a pic of the feed ramps & chamber?
Also can you provide any information as to the make and model of the weapon, caliber, ammunition used and if the problem is occurring with what types of ammunition if you have tried different types, and what magazines have you tried?
Small particles of brass are generally something that happens, which comes from the lip of the casings where the extractor slips around it during the locking cycle. The brass particles will accumulate in several places like the bolt face, ejector assembly, firing pin channel, bolt catch, lower receiver, trigger group, barrel extension, and upper receiver.
So a few pics of the brass shavings and receiver areas, fired brass, bolt face, etc, would help tell the story to see if this is normal or not, and if not then what is happening. So the more details you can provide, will help give us some clues.
CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123