Even shiny electroplated gun finishes seldom have copper underplate. Most gun manufacturers plate nickel directly on steel. Bumper chrome shops used copper underplate.
That's probably true, nowadays, as electroless plating seems to be the most widely used method. You don't see many shiny nickel finishes on new guns... The emphasis seems to be highly polished stainless.
Visits to the S&W and Colt forum will give you references to Colts and S&Ws made in the 70's and 80's that had copper base coats, sometimes visible (on cylinders and front straps) in areas where the nickel plate wore thin with use (or cleaning.) The few I've owned were from that period, including a 6" Python I now regret having sold.
I have talked with people at companies who do nickel plating on firearms and some of them use copper undercoats. In fact, the last slide I had refinished (by Jack Fuselier, in Texas) was, I think, nickel plated over copper.
Jack is now retired, and I can't find his web page (where he described his process). He redid a slide for me, a couple of years ago, and the slide dimensions changed slightly -- and was so tight after plating that I really had to work with it to get it to fit back on the frame and function properly. (It had been a well-fitted and tight gun before the plating, and was not previously nickel plated.) Electroless nickel apparently builds up less on the metal, and is an even-harder finish.