OK, here are some fired cases plus a live for comparison:
No signs of anything on the case sides, no ejector/extractor marks. Primers flatish.
Not chronographed yet but the ADI wonk said 2760 fps out of a AI 26' barrel.
Yesterday we did a rapid at 600m: two serials of 5 rounds in 30s. Unlocking the bolt became progressively stiffer as rifle heated up.
Ran some drill rounds thu. Compared to my Rem 700 it is stiffer to lift the bolt when uncocked.
Examined the bot heads & locking recesses: clear and clean. The extractor assy had
Definitely not common to them all. I suspected a pressure issue until I got to the part of your post that negated that. Do you have appropriate grease on the locking lugs?
some hard cosmoline(ish) stuff on the arm exterior
In comparison, my partner/spotter in yesterday's shoot was finger flicking the bolt of his 6.5CM Tikka though all of it...and he beat me: got all 10x of his rounds off in the rapid without losing sight of the target. I managed 6x & and had to do a total reacquire each shot.