Hi,
You seem to be describing a holster or horse pistol. I have seen quite a few modern made "English" pistols made with octagon to round barrels. Some, perhaps even most, that I examined had barrels on which the octagon section was mostly straight with almost no taper and then any swamp is in the round part. IMO, those barrels look clunky and awkward. Often they are still very thick at the rear ramrod thimble which makes it impossible to create a slim elegant looking pistol. Ideally, whether O/R or swamped round, you want most (up to 75%) of the taper within 3" of the breech continuing slightly to a point just before the rear ramrod thimble or wedding band and then thin almost straight until just before muzzle, then a very subtle flair at the muzzle. Most horse pistols were larger caliber than 50 and ideally you want one with thin barrel walls.
dave