If the rust is bad enough, you aren't going to remove it without ending upneeding a re-blueing job.

Very light rust that is nothing much more than adusting on the surface of the barrel can be removed by lightlyrubbing (not scrubbing)down the barrel with 0000 steel wool and a good gun oil.

Inside the barrel, if it's light, clean with patches "wetted" with solvent to remove loose rust "dust" then follow lightly with a lapping/polishing compound, if patches and bore brushes aren't enough, then reclean as normal. Any rust that has heavily pitted the rifling may need a smith to evaluate the condition and see if it's bad enough to warrant rebarreling or rerifleing (to a larger caliber).

Then again, someone else may know better and be able to help.