You do not have to plate chrome over copper or nickel. Hard chrome in shotgun or rifle barrels is electroplated directly onto the steel bore. The plating is very thin.
Decorative chrome plating on things like car bumpers or motorcycle exhausts is typically plated over a much thicker layer of copper plating, or copper and nickel. The thicker plating fills in surface imperfections, and gives a smooth shiny surface without tedious labor intensive polishing of the basemetal. Shotgun or rifle bores don't need this because the internal finish is much smoother than something like a raw steel bumper out of a stamping die.
The relatively thick deposit of copper applied in a decorative chrome plating process also wouldn't hold up well in a firearm bore, and would cause a much greater reduction in bore size than a very thin direct plate of hard chrome only. It is necessary to make rifle bores slightly larger due to the slight reduction which occurs during the plating process. Especially with a rifle bore, it would be very difficult to maintain an even thickness the entire length of the bore if an under-layer of copper was applied. Naturally, such variations would cause problems with pressure and accuracy.
