Abstract: A ball having a variable volume cavity therein is integrally machined on a haft which is rotatably supported between two bearings. The ball is maintained in intimate sliding contact with a hopper seal at all times. An air-operated actuator is used to rotate the shaft and ball 180.degree. to place the ball in its load and unload positions. The variable volume cavity allows volumetric adjustments to be safely made even when dispensing sensitive explosive powders, such as lead azide. A hopper containing the explosive powder supply is bearing-mounted above the ball and funnel on two vertical shafts. The weight of the hopper maintains intimate contact between an electrically conductive plastic seal insert in the bottom of the hopper and the surface of the ball on which it rests and prevents leakage of powder during fill and discharge of the ball cavity.