I'd almost suspect a low charge to air detonation from the looks of the rifle ( pieces )
I doubt I'd try to do reduced loads with a double based powder with that much case capacity...
you are very lucky... I'm "exprimenting" with lots of powders, loads & calibers right now, & honestly the light charges scare me more than the heavies... I only hope if I should expirience such as yourself, that I'm as lucky as you were...
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