Flash Hole Uniforming
Deburring and sizing the inside of the flash hole so the primer flame reaches the powder charge the same consistent way in every case.
Flash hole uniforming uses a piloted tool to remove the burr left inside the flash hole when the case was manufactured, and on some tools to bring the hole to a consistent diameter. Punched flash holes often leave a ragged lip on the inside of the case head, and that lip can deflect the primer jet unevenly from one case to the next.
By deburring the inner edge, the reloader lets the primer flame enter the powder column the same way every time, which supports more consistent ignition. Like the related pocket work, it is a one-time preparation step performed on fresh brass and often paired with the rest of a first-firing prep sequence.