a spool or mushroom pin which has a portion of its end machined to a smaller diameter than the opposite end. It is typically used as a top pin to improve pick resistance by some manufacturers of high security cylinders.
Ball Bearing: a metal ball, usually made of steel, used in the pin stack to accomplish some types of hotel or construction keying. May be used by some lock manufacturers as the bottom element in the pin stack in one or more pin chambers. Any metal ball used as a tumbler's primary component.