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From Amazon: BUY NOW: Gauss Gun
Gauss Gun: acceleration of steel balls through magnetic potential energy and cascading collisions. Four neodymium magnets provide the potential energy that is converted into significant kinetic energy of the end steel ball. The initial configuration has two steel balls on one side of each magnet- an incoming ball on the opposite side accelerates and impacts each magnet in turn sending each loosely connected second ball off with greater speed.