I think that may be key, given the offset firing pin mark. Case mouth too fat. If it were just right, the gun would close all the way (unless it didn't like the particular bullet shape (still a possibility) or unless the bullet were seated too long for its nose profile). If the case mouth were too narrow, the gun would close completely and the cartridge would headspace on the extractor hook and fire, the way so many seem to do, but it could also cause high pressure if the chamber were at the short end of the range.
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