What Redneck said. Have done it but no 1/2 inch drill so I use a 3/8 dowel and 320 wet-or-dry. Also I don't worry so much about the fit, when you spin it up it acts like a flap wheel.
The idea is to knock off gunk and surface rust proud of the metal, not grind out pits. You'd have to work at it to hurt the chamber.
Gauge? Just go at it until the chamber looks shiny, doesn't take long at all, and go out and shoot it to see if it extracts smoothly now.
