I use Birchwood Casey Lead Remover & Polishing Cloth . . . it does an outstanding job and I use it regularly for removing rust, lead, copper, and other types of fouling. Just cut patch size pieces and work thru the barrel several times using a tight fitting jag. Here is what the package says:
"Brings back original luster. Quickly removes leading, burn rings, carbon residue including copper and plastic fouling, rust and tarnish. Excellent for stainless steel, nickel and most metal surfaces. Great for use on handguns, rifles, shotguns and muzzleloaders. Can be cut to size for removing unwanted residue inside the chamber, cylinder, forcing cone, bore and choke. Cleans and polishes metal, wood, glass, plastic and porcelain surfaces – ideal for sporting equipment, home and auto use."
Costs about 5 bucks.