Datum Dial Kit is easy-to-use ammo measurement system gives you the data you need to refine your case sizing operations. This complete kit includes one Datum Dial tool for checking cartridge cases, Both bullet/Cartridge dials for use with 17 Cal, 204, 224, 243, 257, 264, 277, 284, 308, and 338 bullets, laminated chart showing common datum diameters, instructions and storage box. (A caliper is required.) It provides a relative measurement between the base of a case to a datum point on its shoulder angle. May be used before sizing operations to find the optimal headspace for your firearm. And after sizing operations to check the setup and consistency of your reloading process.