Copper is used in cosmetic chrome plating to give a nice subsurface to polish. If your substrate doesn't have a mirror finish, your final finish won't either. Nickel is added over that giving the silver finish, chrome is actuaslly clear and protects the nickel. Hard chrome finishes are simply chrome over the bare metal. There is a patented method used mostly in aircraft cylinders, but I'm not sure how much different it is from what's used on firearms. I know there is an etching process to pomote lubrication, gives the cylinder a crazed look.