Mike,
Im very interested in knowing more about your gun.

Looks like the model Robust by Manufacture D´Armes et Cycles de Saint Etienne.
In that case the rifled barrel was called "Supra" it was intended for shot ant close range (10 meters) and ball up to 100 meters.
This is ap icture of the Ideal model but Robust (and Costo?) have it too.

IIRK Costo was the model sold in U.S.A..
Do you have pictures of the marks under the barrels?

Thanks
Martin