A rupture in the red marked area is different than a case head seperation. It may look almost the same. When a belted magnum cartridge is headspaced on the shoulder, by controling the bump when FL sizing, the area right in front of the belt can become unsupported in the chamber. Neck sizing may have the same effect. If the firing pin strike does not move the belt forward on firing,while setting the shoulder back at the same time, the brass directly in front of the belt becomes unsupported. After a few firings, the case may rupture. The amount of .015" is the maximum possibe in a loose, sloppy chamber. Not a common occurrence. Seen this problem in a factory chambered 7mm Rem. Mag. The 30-378 weatherby magnum has tigher SAAMI headspace specification, less slop, & should not have this problem.
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