The gunsmith that showed me how to do this used a lead sphere of about .5" diameter with a brass screw run through it. He called it "ball cutting". The procedure was to chuck the ball's screw in a drill press, then swing the press table out of the way and bring the muzzle up to the slowly spinning ball (charged with lapping compound). Then move the rifle in a cone shaped pattern while letting the ball do its thing.
I tried it on an uncles Savage M99 that one of my cousins had been cleaning from the muzzle and egged out the crown. Recutting brought groups from 4" to 5" to about 1".
jim
