As Mike pointed out, Silver mounts were used on lots of high end English sporting guns during the Georgian period. My understanding is that silver mounts on London made guns were usually (not always) hall marked by the maker, who in turn might provide mounts to a number of gunmakers, hence the appearance of identical but rather ornate sideplates and guards on guns that came out of the shops of a number of different gunmakers.
Silver mounts on American longrifles are rare. As I recall Kindig shows at least one but I can't remember the era - I think it was post 1800.
I have seen references to Lt. Vincent Hobbs, a militia officer from southwestern Virginia, being presented with a "neat" silver mounted rifle in return for his tracking down and killing the famous half Cherokee/ half Scot Robert Benge in 1794 and putting an end to his raids into Virginia , Tennessee and Kentucky. I sure wish that gun still existed today.
Check out Gary Brumfield's silver mounted Virginia rifle in progress on his website - it's really cool.
Guy