I am glad to hear you got it figured out, firing pin protrusion is not something I would have thought of either. However it does make perfect sense, if the firing pin were riding in the fired primer dimple it would inhibit the case from being ejected cleanly, and the issue couldn't be replicated by hand because the sear was holding the pin back.

That is an unusual problem but a great learning experience.

Seems that the problem would have been present in the 7.62x25mm configuration if all part were equal in that build.
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