What you are seeing is not plating, just steel in-the-white.
I assure you that CrMo barrels are CrMo through and through, and stainless barrels are stainless through and through. There are no less than 20 cuts on a barrel from solid roll blank to finished product, and therefore no way whatsoever to maintain any type of plating. Yes, there are metal plating operations, but they apply metals like Nickel and Copper that readily dissolve, steel (iron) does not dissolve into solution like that.
Any 'blacked' stainless is a polymer type coating applied to the surface like paint. Nickel can be colored to take on a very dark finish and then be ion bonded to metal and polished to make a shiny smooth dark finish.