EE WAS REPLACING CAM IN SPRING GUIDE. A BENT CASTING WEDGED AGAINST THE SPRING INSIDE THE DEVICE. WHEN THE PUSHER BOLT WAS BACKED OFF PARTIALLY THE LINE UP PIN WAS REMOVED BECAUSE IT WAS LOOSE IN THE BORE. WHEN HE TOUCHED THE END CASTING IT RELEASED THE SPRING TENSION PROPELLING THE HOUSING STRIKING HIS HAND AGAINST A WORKBENCH VISE.