stonehog Wrote:... some of the other rounds near this one had a slight line developing around the case head. I will post pics later if I can get a good close-up.
Based on all the great posts in this thread, I'm thinking your 12" just doesn't dissipate the pressure to the 5700 psi spec (mentioned in the Bill Alexander quote screenshot). And this was impacting all your brass, but it was fine at the start. And as your rifle heated up more and more, the issue became exacerbated, and as that line or ring near the head started forming, it got worse and worse with each shot. Add to this the thermal expansion of your chamber itself. Eventually...kablooie...at round #41.
Edit: Also, you might want to start at minimum load and work up to 28.5 gr. Out of an abundance of caution and such.