I'm not an Engineer or Metallurgist, but IMHO, this mischief was caused by extreme high pressure gases (due to over-load, under-load detonation, bullet/throat problem, compromised cartridge case???) blowing out the case head and allowing the gases to enter the action. (Metal fatigue or flaws may have contributed, but, probably not necessary or root cause).

When rimless cartridges and their rifle actions 'blow up', they look kind of similar, with top of receiver ring and extractor gone, and other wreckage somewhat restrained by wood.

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