I have an original flint VA rifle, probably circa 1800-1825, with a tapered barrel. I am not sure it would fit "mountain rifle" description since it has patchbox/sideplate/buttplate etc but it is all iron mounted. I had not seen a tapered barrel on this an rifle this early and wondered if it was common. Wallace Gusler told me that they were not un-common in the western area of VA. I didn't ask about other areas.
Dennis