boil them........when Im rust bluing barrels after degreasing I boil them, then apply the first coat of rusting agent.....after rusting I boil them again, card, quick dip back in boiling water to make sure they are clean....take out of hot water and they dry quickly....still plenty warm, reapply rusting solution....repeat , repeat , repeat....
If you are browning instead of bluing the barrels I imagine a propane torch or just lay in the sun for a while in the summer would work.....Id be very careful with the torch though....
