I just figure it's likely, if they were trying to rehearse the right timing and movements for the scene, that there would be a person behind the camera looking through the viewfinder to make sure it was all in frame.
Edit: I just realized there's an easy, cheap way to avoid this issue in the future, besides just following the normal filmaking armour's regs.
Any scene done with a gun pointing at the camera should use a 90 degree turn in the viewfinder, so crew are not directly in the line of fire if there is a mishap.