JB Bore Compund on a cloth patch wrapped around a brass brush. One patch gun oil.. one patch JB. Keep rotating each one for several patches using complete up & down strokes... avoiding short strokes. Give each patch around five strokes up & down with JB's... only a couple with the oil. After every oil patch, use one side of a dry patch to remove excess oil before starting another patch with JB. You want to leave a little trace of oil in the bore for the next JB patch.

Then switch to a regular gun solvent to remove the JB from the bore.... even if a few traces of rust remain. Oiling the bore & leaving it soaked for a couple months usually loosens the rest of the rust.... provided it's not rusted too bad with some pitting.

That's the method I used to remove rust in a garage sale rifle I purchased. Oil her using two patches with Rem-Oil, Birchwood Sheath or Break-Free CLP type gun oil when finished.... everything with a bore brush. No dry patch to remove excess oil... just stand the rifle on the muzzle for a day or two to release any drips... then turn her over for storage without the nipple in it. In a couple months, see if solvent & new oil turns the patches white. If not.... give the bore another JB treatment.