That's rust and maybe your bluing failing. Try 000 steel wool and be gentle, don't remove the finish. I would take the whole gun apart and look for more rust.
If it's bad you can plug the barrel with wood dowels, boil the whole barrelled action in distilled water converting red rust into black rust, and "card" the rust off with a carding wheel, which is a lot like a buffing wheel, then refinish. You could paint but I recently cold blued a gun and it came out nice.
There's an at home process called cold bluing. Pretty easy. You'd scrub all the old finish off with a scouring pad and gun cleaner, wash with soap and water, soak the action in gas or acetone over night, handle with gloves/dont get any oil on surface, hang like painted parts, and apply this cold bluing chemical, let almost dry, and wipe off with a clean rag. Preheating before application with a heat gun helps, not hot to the touch just warm. Buff gently with 000 wool between coats, don't remove your surface. Makes a very thin layer of rust that inhibits destructive rust. Most people do 3-6 coats. When you're done spray cold water from the garden hose on it to help close the pores of the metal. Takes an evening.