These rods are generally used on a chambered barrel. They stick out of the barrel further than a Range Rod. This is because of the 3 degree taper. This gives a longer section of rod for you to be able to get a test dial indicator on the rod. Indicator rods use removable bushings just as reamers do. Comes with one bushing. Bushings by Pacific Precision, JGS, Henrickson, Francis, Forgreens, and Elliot will fit. If you chamber with the barrel held by a 4-jaw chuck and the muzzle extends through the spindle bore, and are trying to indicate the bore in for a rechamber, you found the answer.
The rod centers itself with the pilot bushing and a 3 degree taper ground onto the precision rod. It works best on a chamber with less than 50 rounds. As the throat wears, it is not concentric with the bore.