Hi Tim,
Most of the commonly available octagon/round barrels are of Spanish form, meaning they have 2/5s octagon and 3/5s round with a wedding band transition. Trade gun barrels varied more greatly with some having very short octagon sections and gradual transition to round with no wedding bands, some had octagon to sixteen sided shape to round, some had 2 bands of some sort. It varied. I don't think you need worry all that much about what barrel you use unless you are really trying to copy an existing original gun.
dave