The RCBS Case Master Gauging Tool is a must-have for the serious reloader, because it will help you to achieve the maximum level of accuracy in creating reloaded ammunition. This gauging tool helps to measure not only the concentricity of your case necks but also the thickness, uniformity, and bullet run-out as well. Use the RCBS Case Master Gauging Tool to ensure that all of your reloading equipment is properly set up and calibrated to deliver the highest level of accuracy possible. As an added bonus, this gauging tool can also detect separation of your case heads before it is evident on the outside of your cases.