Primer Pocket Uniforming
Cutting every primer pocket to a uniform depth with a square, flat bottom so primers seat to a consistent depth across a batch of brass.
Primer pocket uniforming uses a small cutting tool to square the bottom of the primer pocket and bring every pocket in a lot to the same depth. Factory brass varies from case to case, and a shallow or domed pocket floor can leave a primer sitting high or compressed, which changes how hard the firing pin strikes it.
By making the pockets identical, the reloader gives every primer the same seating depth and the same anvil-to-floor relationship, which helps ignition behave the same way shot to shot. It is a one-time prep step on new brass that many precision handloaders fold into their initial case preparation routine before the first firing.