So an update... Learned some stuff in the process, but in the end, I'm not really sure I've "solved" anything. And I'm not certain there's anything to solve.

A recap...the players:

"Problem Rifle": Makes marks on spent, ejected brass. Does NOT make marks on unfired brass.

"Rifle C": Known good rifle, used for testing. Shoots everything fine with no issues.


- The Problem Rifle was checked for headspace with 5.56 gauges. Bolt closed on Go-gage...PASS. Bolt would not close on No-Go gauge...PASS.
- Problem Rifle BCG was put in Rifle C and worked 4.0, no brass marking (this had me puzzled after it passed the headspace test).
- Rifle C BCG put in Problem rifle and worked 4.0, no brass marking.

Conclusion: I have no idea. At this point I can't find anything wrong with the Problem Rifle. I can swap out the extractor, but it didn't make marks when put in another rifle. Unless someone has other ideas, I'm at the point where I'll just keep an eye on it but keep shooting.