The original poster still hasn't answered on the gun makes. Are we dealing with modern or vintage pieces? If modern, why would a mfr. silver plate a gun receiver when there are much better looking, more durable, and cheaper finishes that look like coin? If the guns are more of a vintage type, the so-called coin finish is usually just matte polished steel. In that case the steel may have some alloying metals that make it less resistant to oxidation. For example the Chromox alloy used by A H Fox. In either case, modern or vintage, I think the resident alchemists here should forget about the black tarnishing common to silverware. I really doubt the receivers on many high grade doubles were silver plated. What are the odds of finding two with the same problem at the same time? Polished steel will definitely turn black - that's the bluing process - although it usually involves boiling & high heat to convert the ferrous oxide into ferric. I suspect another reaction is at play here but without knowing the type of plastic, felt, oil or rust preventative compound used beforehand, etc. we're all going to be shooting in the dark.
