Although I have no personal experience with this product I have heard great reviews...If you check the link there is a video in the middle of the page describing how to use it for the application you want.(barrel rust removal)
http://www.bluewonder.us/BlueWonderGunCleaner.html
Blue Wonder™ Gun Cleaner will also remove rust from the exterior finish of a gun - and it will not harm or affect the existing bluing! Rust is iron “oxide” and it is affected in the same way as the copper and lead oxides. Simply use the cleaner with “0000” steel wool or a soft brass brush and the rust will be removed and the bluing unharmed.