Yes! Awesome. Great info.
My summary: it seems even the Mk 262 stuff wasn't fragmenting as well as they'd like out of the 10.5" barrels so they went to an expanding bullet "optimized" for SBRs with a lower velocity expansion threshold hold. Great.
My understanding is that while Mk262 might offer better terminal performance than M855, it is still more of a match round than a terminal performer. Saying something does better than something that sucks isn't really an endorsement.
Feedback from some former coworkers running 14.5in 416s/M4s is that they are getting better results with the newer M855a1 than they had with Mk262. One went so far to say that he was 'less than impressed with the Mk262' when it came to putting people down.
Take that FWIW, very small sample.