I see several potential issues
CCI primers #35? Those will never get good ES bc too inconsistent ignition. Need RWS, only one source: Dale Arenson Ten-X accuracy. Going for $1 each these days.
Twist is way to fast for the 500 A-tip, that's why they’re flying apart. 12 twist or thereabouts would be ideal something like start 12.0 with gain up to 11.25 or thereabouts if you’re into GT, or just go with 11.5.
Early Barrett brass made by PPU, or current RWS made? I’d be shocked if RWS necks were splitting. Like shocked esp if being properly annealed. Unless chamber neck specs too wide. See:
https://www.snipershide.com/shooting/threads/416-barrett-ammo.6851691/
I run RWS brass/primers for my 50. Massive difference in SD. Virtually eliminates vertical dispersion at distance. I measured internal case capacity of 20 RWS “50 Browning” cases and the standard deviation in grains of water was like 0.2 SD! It was like they were made in the heavens, the internal consistency was so good. I have never seen anything near as consistent in internal volume. It beat the pants off Lapua 50 BMG, SD for those was like .4. Respectable for sure (that means 95% of the Lapua are +/- 0.8), but RWS wins by a long shot.
Before I fixed my velocity consistency issue by going to RWS primers, I also was weighing the CCI primers to try to address the inconsistency. It didn’t make a damn bit of difference. Those primers just suck balls. And bills. I.E., $ bills trying to get the consistency you expect.