The upper is a complete BCM 11.5in upper running a DD bolt, lower is a G&R built BCM lower. I changed the receiver extension to the Vltor A5 and have tested both A5A3 buffers, as well as the standard A5 buffer. Both have this issue. am having the following issue every few rounds:



It almost looks like a double-feed, but it's actually that the previous round is failing to completely extract and the live round is feeding under it.

I've tried multiple magazines, all exhibit the same issue.

The fired brass is indented as pictured below, but I didn't know if that was a symptom of the case getting slammed together, or related to whatever is the root cause.



Curious as to what would be the next test to run:
1) Test a different Complete BCG.
2) Test lower against my 16in BCM mid-length upper that runs fine on an identical lower that I have.
3) Something else?