GunKote is cheap and is supposed to be effective. If your oven is not big enough to bake the barrels at 300 deg F, you can cook 'em in your BBQ by laying a steel plate over over the grill (or use the iron griddle inserts if you have them). Screw together a small jig with iron pipe to support the barrel assembly above the plate/griddle, preheat the BBQ, and go for it. If I recall corrrectly, the metal should be at 150 F or something like that before you apply the GunKote. We can get that temp around here with 30 minutes in the sun.
Another option might be to prep the bbls and etc. with fine sandpaper and then have them powder coated. I'm planning to GunKote a badly rusted saltwater 870 real soon. Had it bead blasted and blued a few years ago, and it rusted faster than it did with the original finish. You could quite literally watch the metal turn red/brown while you sat in the blind.
