A material reduction machine includes a frame, a drum housing mounted to the frame, and a drum that is mounted for rotation within the drum housing. The drum includes a circumferential wall and a knife that is mounted on the drum. The machine includes a plurality of interchangeable belly bands, each of which is adapted to be removably attached to the frame so as to form a portion of the drum housing and to provide an inner surface that forms a belly band arc portion that is concentric with and of a larger diameter than the circumferential wall of the drum. A first interchangeable bellyband has a smooth inner surface, and a second interchangeable belly band has a plurality of flow interrupters spaced along its inner surface.