if you get it down to bare metal id paint it like a car......sand it with some light sandpaper....rub it down good with rubbing alcohol....and as soon as the alcohol drys prime it.....as long as you dont get moisture under the primer you should be fine....after a coat or 2 of primer hit it with your favorite paint a couple times....maybe a camo job...whatever you wish...id stick away from shiny metalic glossy colors....just wont look right on a gun barrel lol.....someone will have better advice im sure......ive never painted a barrel...