Okay, some things to add. Did a final zero and ran another 20 rounds; 100% function with the LMT carrier group and the new set of rings. Bolt didn't lock open on the last round.
Thinking about it on the way home; I'm running the rifle buffer that I'd been using with the 20" Colt.
Would it make sense to go to a carbine buffer? I have an ummarked carbine buffer in my spares box; presume that is the lightest version?
Moon