Quoted:
It it possible that the bolt is somehow installed backwards?

Quoted:
If the rim is still being held by the extractor,

Then either a bur on the extractor claw or the rim relief cut  not correct on the extractor that did not allow the spent case to pivot off,
The ejector bounding in the bolt face channel,
And/or the face of the bolt retracting back past the back edge of the ejection port window (bolt face should stop 1/8" to 1/4" in front of the back edge of ejection port   window when you pull the charging handle all the way back).

Quoted:
Since you know that the extractor has burs and causing all the brass shaving problems, then clean the burs off the extractor.

With the extractor removed from the bolt, Run your fingernail down the bottom of the grabbing claw, and use a file to remove the side edge burs (leave the extractor claw itself sharp).

Just below the claw 0n the extractor, is the rim relief channel for for the rim to seat into when the claw snaps over the rim, and on both sides of that channel, more milling burs that may need to be clean up as well.

Think of it this way,
When the bolt goes to unlock, the spent case is still pressure bound in the chamber, so the entire bolt/extractor/ejector has to slip/rotate around on the back of the case rim before the lugs can unlock and spent case pulled rearward out of the chamber vi the bolt at that point.   So during the unlock rotation of the bolt on the spent case, the more milling burs that are going to scrap on the case rim, the heaver the brass flake fouling problem is going to be.