Octagon barrels have flat sides. which are referred to as "the flats" In other words, how thick is the barrel from one flat to the opposite one?

This is obviously a percussion target rifle, either a Schutzen for offhand shooting, or perhaps a benchrest gun. I conclude this from the fact that it has a hammer & nipple arrangement for the back-action lock, double-set triggers, an aperture rear sight and what appears to be a target sight on the front, as well as having a false muzzle.

It DOES NOT have Whitworth rifling-Whitworth rifling does not have grooves between the hex surfaces! This is probably a slug shooter rather than a round-ball rifle, due to having a false muzzle. Measure the twist. I would date CA1845 to perhaps 1860.