The Hornady Flash Hole Deburring Tool is designed to be inserted into the case mouth and uses the web of the case as a safety stop. The tool is then given a couple of turns and it produces a small chamfer on the inside edge of the flash hole. This operation removes any burrs that might be present surrounding the flash hole insuring proper, uniform ignition. Flash hole deburring is a one-time operation and best used on brand new cases.
Big Book Catalog, Issue:66, Page:510
Sinclair, Issue:3B, Page:012
Sinclair, Issue:3B, Page:015