I would anneal some cases and test them with the 138 load. Neck tension is a big factor
in these cases. The brass i sent you was necked from 408 brass and fire formed and prepped. They should be annealed after the first fireing as the necks get hard going through
the forming process. I had a customer that experienced similar results and it was magnified
at 1000yd shooting. He couldnt figure out why one day he would shoot lights out, 4-5" groups and the next time he went out the groups were larger and so on. The necks were getting hard from forming and reloading, his groups were mostly vertical. He annealed the necks and the same load went back to the original group size. Some longrange bench shooters anneal everytime they load, i would say these cases every three shots should suffice.
Your chamber neck size is .372 a loaded round should measure .368 or very close. The reamer is set for .015 max neck thickness
I agree with others, you should shoot the group in one setting as fast as possible, dont let the gun cool as it can change lots of peramiters. The carbon barrel cools very fast internally so fire them quickly. When i tested it that is what i did, i fired 3 shot volleys and dont let a loaded shell sit in the hot chamber too long. My chrono had them a little faster than yours is showing, 3300 was average with the 138 loading.
Measure the base of the case, ahead of the extractor cut on a case you havent shot before. record this...should be around .634 after firing it measure it again in the same spot.
.005 - .006 is the norm for a full load expansion. .640 would be a top load.
If you get a chance see if you can start to stretch the distance out, 500 - 700 yds
Another thing to try with the boron coated bullets is to take a soft cloth and wipe them off before loading to remove any excess boron. Very slick bullets can cause the ES to show up on the chrono. Coated solid bullets can have extreme spreads.