I would recommend using something like JB pasts or ISSO paste with a 6 or 7mm nylon brush and a 12 ga patch and polish it out. If the rust is back off the shoulder, just use a larger caliber bore brush. It will take some elbow grease and time, but it will polish it out nicely.
If you can find it, RB-17 is a miracle cure for light rusting. Use it and some super fine steel wool, and you can remove some serious rust up to pitting rust with ease. It has served me well with my duck shotguns as they only get cleaned after the season. It has yet to fail me in getting the rust out/off.
