I camo'ed a beater 12 ga. Mossberg 500 with assorted flat black, flat white, dull brown, grey primer, and OD green Krylon. I did the stock and all. Ugly as sin, but has served me well turkey hunting for the past 20 yrs. or so and it isn't rusting. If a get a bare spot I just add a touch more paint. I keep the innards clean and lubed and it works fine.
Did I say it is ugly as sin? It was never a high quality gun, birch (or some such) stock, pressed checkering that didn't align, rough metal work, but it was cheap and worked. I didn't mind it rattling around in the bottom of a duck boat or canoe and it shot where I pointed it. When I decided to try turkey hunting I just painted it up. It is not something that I am very proud of, but it is functional.