The one thing that really works, "Evapo-Rust" pick it up at the auto parts store and plug the barrel so it can not get out, fill the barrel and let it soak for 10 to 12 hours it will eat all the rust out of the barrel. If you have any sharp edged pits you may need to lap. Clean the barrel with boiling soapy water after taking the Evapo-Rust out the oil. no elbow grease is necessary for the process in fact it is best left undisturbed while taking care of the rust for you.