Another option(s) has occured to me: T/C made a gun called the New Englander. It came in .45, , .50, .54, and 12 gauge (smoothbore, with and without removeable chokes). The Greyhawk is another one made with round barrels, and it was made of stainless steel. I recently became aware of called the High Plains Sporter, which was basically a New Englander with a pistol grip incorporated into the stock. New Englanders generally don't go for top dollar in today's market.
So, if you want a round barrel, that is an option. You could just buy the gun and use it as it is, or make the stock you want and fit the relevant parts. If you take the first option, you'd be saving hundreds, and as I and others will attest, the New Englander, though not their top of the line gun, IS quite serviceable, as a rifle or a shotgun.
I suppose you could draw-file an octagon barrel round, but that seems like too much work to me.