Unless it was mechanical failure where the barrel snapped to its lowered position instantly instead of 9 seconds, the whole thing doesn't make sense. It's like a car coming to you at a ridiculously slow speed of less than 1 km /hr and in that very instant, you lose your sense of reflex, sight, hearing including self-preservation and as such, failed to, or simply refused to move.
This sketch posted by someones else earlier compared to the assumption that he was pinned to the wall and not the ceiling of the cabin, makes some sense though. It would suggest he was working (hopefully not mucking around) by straddling the breech/barrel to fix it and things went very wrong. The "pin to the wall" assumption just doesn't make sense unless he was drunked/drugged out, was responding to some dare or the cabin was filled with rubbish which impeded movement.
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