I just got through polishing the chambers on my .223 and .357 Mag barrels. It's a pretty simple process. I just took a fired case and drilled the primer flash hole out to 7/64". Then I screwed a #6 X 1/2" hex headed sheet metal screw into the drilled out case, put a little Clover "Fine" valve lapping compound (available at better auto parts stores) on the case and inserted it into the chamber. Then I used a cordless drill with a hex adapter on the screw head to spin the case in the chamber. A little in and out of the case while it's spinning helps. Remove the case, clean everything up (Clover is an oil based compound) and the job is done. Wal-Mart Carb Cleaner is a good cheap cleaner (it will take the finish off your wood, so be careful where you spray it). Swab it out with patches and oil after doing the cleaner.
Do I need to remind you to go easy with the process. Remember, you're polishing, not making a .45/70 chamber out of a .357!
