Range rods are used for aligning barrels in the lathe chuck before reaming out the chamber. The front of the range rod takes a pilot bushing appropriate to that calibre and is slid into the barrel. The body of the range rod is slightly tapered so that at some point you cannot slide the rod into the barrel any more because it equals the bore diameter of the barrel at that point. The range rod is now sitting in the barrel parallel to, and concentric to the bore of the barrel. The TIR of the rod still projecting from the barrel can now be minimised to centre the barrel in the chuck
By checking the TIR at two points on the rod, it is possible to ensure the bore of the barrel is also axially aligned as well as being centred at the chuck.