I have done a couple rim recesses with piloted counterbores. Turn a new pilot to fit the chamber and grind a slightly larger counterbore t the diameter of the rim recess. If you don't have a toolpost grinder, you can "rig" a hand grinder in the milling attachment to make one. I find that 4 or 5 small cutoff discs(trued) stacked on the handpiece makes a satisfactory grinding wheel. I turn the work backward and the grinder forward, but friends disagree with that. Cover the bed and dovetail surfaces well. Use a piece of sheet metal to block the sparks from catching the cover afire. I always turned the counterbore by hand to prevent catastrophic mistakes, but I can understand setting the barrel/extractor between centers in the lathe with the counterbore in the headstock and a plug with center in the appropriate barrel muzzle. I would still loosen the belts and turn the chuck by hand, rather than under power.