Brand new .300 BLK 8.5" using AAC 7.62 SDN-6, Remington 220 gr. subsonic, 187 gr. Gemtech subsonic and Fiocchi 150 gr. supersonic. Barrel is from Aero Precision. This is the first time I have experienced copper buildup on the bolt tail behind the gas rings and on the middle of the bolt at the raised band just behind the extractor pivot pin. There are also several tiny flakes of copper in various places in the lower. I've been shooting M16 / AR15 for quite a while 5.56 and .223 but this is the first time for .300 BLK.

Any ideas? I've queried Aero Precision, but no answer back from them.

I have a 10" blackout upper with similar copper build up.

Couple hundred rounds of heavy subs through it shooting with a suppressor.

Don't know what causes it.

A burr in the gas port hole in the barrel is most likely raking off bullet jacket material as the bullet passes the port.  It is getting blown back into the bolt through the gas tube and accumulating in the bolt / carrier assembly.   The burr will eventually wear off and you should not see this condition persist.

This.

If you have an adjustable gas block, adjust it periodically or the copper buildup could freeze it up. Supersonic rounds will clean off the burr faster than subsonics.

I experienced a similar situation with a 9" AAC 300BLK upper. I haven't noticed it in a while though.

Mag dump some high velocity .300 BO.

$20-40 fix

Ok, so two votes for a burr. Reflects some comments from other sources. Aero Precision called and said that it was normal for .300 BLK and they would give me the e-mail to their supplier so I could verify with them. Still waiting for that e-mail address.

Thank you for the responses so far. I'll run some mags of supersonic through it soon and see if it clears up. Maybe see if I can find someone with a bore scope.

1st Cav Div, 312th MI, Desert Storm.