Not sure where on the firearm or what on the firearm is rusting, or even if its on blued, parkerized, nickle finish,... But I have had pretty good luck using a piece of hardwood cut straight at a sharp ninety degree angle, then douse the area with a light oil. and hold the wood at about a forty five degree angle, then rubbing towards the rust,changing to a fresh piece of wood very often as the rust is abrasive, and the pieces of rust now stuck in the wood will gouge the surface. Sorry if that description was a bit confusing, tried to make it as clear as possible. I've only used this on blued firearms by the way.