Head separation on rimless bottleneck cases is caused 99.9% of the time by excessive head clearance. Too much difference between chamber headspace and case headspace; the gap between the bolt face and case head is too much after the firing pin's driven the case hard into the chamber shoulder. Anything over .003" is too much in my opinion.
Sizing fired .30-06 cases following supplied instructions with the dies will resize the case close to minimum industry specs. If the rifle's chamber headspace is near max specs, there'll be several thousandths head clearance when the firing pin fires the round; .008" head clearance is not unheard of. The die needs to be set higher in the press.
When the round fires, its front half presses against the chamber walls and the back half stretches back. It's the back part stretching back that weakens the case wall about 1/4" forward of the case head where the brass is work hardened the most from firing and getting sized back down. That cracks after a few cycles and the head separates.
Proof loads with much higher pressures than normal don't cause head separation with cases the right size for the chamber. I've fired hundreds of proof loads in new cases put in chambers such that they've got no more than .003" head clearance and no case head separation at all. There's not even incipient separation felt by dragging a pointy thing inside the case and trying to feel a ridge around the case where it hangs up; no ridge exists.
There's tools available to measure case headspace; RCBS Precision Mic and Hornady LNL Gauge are two excellent ones. Measure a fired case headspace then set the die such that after its sized, its headspace is 1 to 2 thousandths less. Then you won't have case head separation. And a few dozen reloads per case is often possible doing this.
|