Highdrum : are you certain you had the hammer forged c272 brass with the ATB design in the cases you had issues with ? maybe you had the c260 conventional brass cases ?
This was forged c272 brass WITHOUT atb design, 300wsm. Out of 20 cases fired, I had one case rupture inside the extractor cut, and one split a neck first firing. This was with 64gr rl23 215 Bergers @ 2820 fps. I ran 65gr in norma brass for same speed.