I've found a little something among my grandfather's possessions that I don't know what to make of.

It's the barrel to a muzzle loader. .58cal, approximately 42" long. On the rear of the barrel, which would be covered by the stock, the number 81 is visible in the center, with a letter "F" to the right and possibly a small 4 beneath the "F".

Anybody have an idea of what kind of weapon I'm looking at here?