Think I've found the issue with a FTE problem on a new upper I bought this weekend. It had 2 FTE from battery.

1) Somehow a polymer wolf round got loaded with all the brass cased stuff I was using to break in the chamber. Accused the wolf for the FTE.

2) happened once more on q3131 and shot fine the rest of the day, ~ 2-300 rnds total.

Stripped it down and did inspection, found the carrier key loose.
Just got done putting it back together. Found that the MFG did not loctite the screws and did a half assed job of staking the screws.

Could the carrier key have lost enough back pressure to cause the FTE. The key was loose enough that it could be wiggled just by shaking the carrier but could not see light between the carrier and key.

Yes!