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The M16A1 and A2 would sometimes fire a round if the butt was hit hard enough and a round was in battery - a hard fall or jumping out of a vehicle and dropping the weapon would do it. Safety on or off it did not matter. The firing pin did not have a trigger block.

And then there was the "No Discharge" where it would double feed. I never once ran a live fire assault where this did not occur to me, or to one of my guys.