Thanks all.
Red, you have a good point about over-gassed.
Gas port at rifle length, 20" barrel.
Do have an adjustable gas block that is worth swapping out.
I want to stay with 8208 if possible, already use it on 243 and 308.
May look for a less hot primer than WSR.
Also, I tend to clean & over lube the BCG rather than under lube it, fwiw..
Thanks again & will carry the (hot) STMK 69s on my next yote trip.
Worst case (pun intended), the AR chews up brass too fast.
On a related topic, my CZ527 didn't like the same STMK loads, sprayed them +3 inches,
probably inadequate twist rate 1:9, vs AR at 1:8..