Range rods use removable bushings just as reamers do. Comes with one bushing.Bushings by Pacific Tool & Gauge, JGS, Henrickson, Francis, Forgreens, and Elliot will fit. These rods are for new barrel blanks. This is used to align the barrel bore with the axis of the lathe. If you chamber with the barrel held by a 4-jaw chuck and the muzzle extends through the spindle bore, you found the answer.
The rod centers itself in the bore with the pilot bushing at the front and a 1.5 degree taper ground onto the precision ground mandrel behind the bushing.