While there is not a lot of evidence that flash hole uniforming is an asset to accuracy, there are a lot of differing opinions. Consensus does exist to some extent that uniforming flash holes doesn't hurt. The process involves using a tool that has a drill/reamer tip just slightly larger than the existing flash hole. A few twists generally accomplishes the task and some of the tools available are also equipped with a chamfering cutter that serves to bevel the inside edge of the hole and remove burrs at the same time. To offer an idea of the concept, a picture is available using the link below of a magnified flash hole from the factory (compliments of Ken Turner).
