My understanding is that octagonal barrels were easier to forge into shape and then to finish with fewer tools/equipment - finished by drawfiling by hand. Round barrels are shown in period illustrations being ground round on huge water powered grinding wheels in large commercial works which were beyond the means of many/most American riflesmiths. As a result of the equipment available, most early American made rifles would have hand filed octagonal barrels while most early round barrels would have been imported from large works in England. Once powered machine tools came in, any shape or taper could easily be made but a round, tapered barrel became dominant as it was both lighter and cheaper than any other shape. As mentioned in an earlier post or two, both rifles and smoothbores were/are found with octagon to round barrels which, due to the round part, I believe may largely be the products of commercial barrel makers rather than small shops - just as most locks were made by specialists rather than individual gunsmiths.