One thing I don't think has been mentioned: a dead soft brass case head. Could the case have missed a step in it's making...the hardening of the head area? Annealing is done to the forward neck section, but a soft head could rupture, and then the flowing hot gasses will melt their way for further rifle/case destruction and schrapnel projecting.
Just like the O ring on the solid rocket boosters: once a leak starts, hot gases erode for massive failure.
Just a thought. Even a standard pressure load could make a 'too soft' case head melt and flow.
I have been pushing the Mauser case head to 67kpsi, the belted magnum case higher, .223 case head to higher still, and the 6mmBR case head even higher.
If the distance between the extractor groove and the primer pocket is too small, the pockets get loose with less pressure, like with the 10mm and 25acp handgun brass.
But in rimmed cases, the brass should go much higher than with rimless. With rimmed and 6mmBR, the primer pocket should not drive the max pressure. I have tried every variation of this many times, I thought.
But in Oct 2012 I built a 257 Roberts Ackley Improved Rimmed on a Uberti 1885 Win falling block action. I worked up loads at the hunting site with presses in my vehicle.
I was going to use H4895 with 115 gr Bal Tip moly bullets, but the case heads started failing [loose primer pockets] around 60,000 psi per Quickload.
I switched to H4350 and shot the deer with a few hundred fps less than I had planned.
Norma had sold me dead soft case heads at $1 each.
This was not the "C26000 brass (cartridge brass) Temper - H06 Tensile yield strength - 65,300 psi" that Scott Sweet uses and assumption for Von Missed equations he uses to calculate the max pressure for each cartridge.
I have some other brands of 7x57mm loaded ammo on order that I will test for pressure.
