Maybe it's the result of a small sample size, but every ND that I have had reason to investigate, in which the trigger was not pulled, happened exactly the same way.

Firearm with chambered round, and hammer at rest, falls muzzle first onto a hard surface, inertia causes the hammer to partially, but not fully or half cock, then hammer falls igniting round.

Guess pretty much the same physics butt first.

Yes, that is what half cock is for, esp. when moving in terrain after loading chamber and or firing.

Glad you are ok.

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