Use a bit of engine oil with fine wire wool, you dont need anything other than that if the rust is still there get the barrels refinished. Also get some engine oil and heat it gently add a spoon of vasaline and melt this in, keep warming it untill it has mixed together then leave it to cool, coat this on your barrels and then wipe off it will leave a protective sheen on the surface, better than wd 40 or anything else.