I picked up 3 rifles in a package deal recently, and while the seller said that they were fair, an SKS and a full wood Swedish Mauser both had rusty bores. The solution that worked great was electrolytic rust removal. I made a solution with washing soda and used a sacrificial anode made of rebar and a piece of round bar that fit inside the bore. I wrapped the round bar in electrical tape, leaving parts of it open so that the electrons could flow. It took 12 hours of current, but both bores are now shiny and bright. Haven't had a chance to shoot them yet though.