Creedmoor Sports Primer Pocket Probe for Dial Indicator Having a clean and uniform primer pocket helps to ensure consistent primer ignition by ensuring the same amount of primer "crush" every shot. This makes the speed of ignition more consistent, and uniforms lock time resulting in more consistent velocities and more uniform groups. Primer pockets are typically cleaned during case prep through case cleaning. A clean primer pocket is one step closer to becoming a more uniform primer pocket. Primer pocket uniforming cannot be done with a simple pocket scrapper, you must use one of the many tools which cut the bottom of the pocket to ensure a squared and uniformed in depth pocket. The primer pocket probe measures the depth of the primer pocket and is best done after case sizing. This add on tool for the Granite Stand is indispensable when you are prepping cases for the utmost accuracy. Compatible with any dial indicator that has at least .250in of travel, and a replaceable probe threaded 4-48.