You need to send us more pictures - this looks like a VERY interesting gun. The engraving and carving may help the real experts.

Love that "checkering".

First throw-a-dart: the "a Vienne" on the right barrel means "of/from Vienna", in French. The word on the left barrel, which I can't make out, is most likely either the maker's or retailer's name.

Second throw-a-dart: I'll guesstimate sometime in or before the 1860s-1870s, first-generation percussion and not converted from flintlock. Or, if it was converted, they hid it well.

To all appearances, a high-quality piece in very high condition. Might have been made in Austria for a Francophone market.

fiery, dependable, occasionally transcendent