An extra, clean barrel is always nice to use for the polish stage as it eliminates the careful clean up between the grit and polishing stages making it less likely for contamination.
203 AR 6 barrel complete with lid and band
6 3/4″ diameter and 3 7/8″ tall (interior dimensions)
8″ diameter and 4 3/8″ tall (exterior dimensions)
(two of these barrels can be put on AR12 or Model B tumblers to have two stages going at once!)