I just bought a barreled receiver recently not for a build just to look at and measure. It had a lot of paint of all things and some rust. Some of the paint removers are very toxic so I used brake fluid to get the paint off.
Important is do no harm, so sand paper, wet or dry paper and abrasive scotch brite would be a no/no for me. Penetrating oil and paper towels can do a lot, on heavy rust maybe fine steel wool with lots of oil. There is a nonabrasive scotch brite made from blue plastic for dishes, not sure how it would work on steel parts with oil for flushing it out. Might be worth a try.
Picture of the stock will help the others give the best advise. Oh and we like to see Krags anyway, even if it does not help us give advice.
One more thing, Welcome to KCA.