Could be the very air as noted depending on the "silver" coating.
More likely to be glue or process used on the leather. Ask anyone carrying a silver item in a Coach purse...
A clue is usually that the problem generally occurs more on the side away from that resting on a surface.
IN GENERAL, a light oiling should help prevent this.
One test might be to take the gun out and replace it with a piece of silverware for a week or so.
Might also suggest wrapping the guns in sleeves made of Pacific cloth which many of us do for transport in leg o'mutton cases.
