Gently, slowly,and consider if the "game is worth the candle". Dent/bubble removal is really an art. I use expanding mandrel and/or hydraulic dent removers, depends on size and depth and location. NO heat. Small ballpeen hammer and GENTLY. Usually end up draw filing barrel to smooth exterior. Gently hone internal barrel to smooth any high spots. Re-rust blue to finish. Good Luck! Dr. BILL