Alright, I replaced the extractor spring/insert and lubed up the BCG without cleaning anything and put everything back together. I'll see if the malfunction comes back in a couple hundred rounds. If not, I'll put back the old spring and the FTEs should come back.

The left spring is a brand new one before installation. The middle one is the suspect spring that's taken a set after installation and under 300 rounds fired. The right one is the stock spring/o-ring (no insert) that came in the C4 BCG.



Here's what the chamber looks like. There was a loose piece of brass shaving that I was able to blow out with my mouth. I don't think that had anything to do with the FTEs because the last rounds fired in my shooting session didn't malfunction.