Octagon was the standard in ML other than military muskets.

I once swamped a barrel for a muzzle loader that I (finally got) built. Vall comments on the finish bearing no evidence of all that hand work of shaping the metal to octagon. I can say, with less metal to remove on mine, that those guys were persistent dudes. And, in agreement with the finish being so good on theirs, I spent about as much effort 'cleaning it up' to look good. 
Well, almost good. Never again! Nowadays, I never look further forward on an old flinter than the buttstock. Don't want to be reminded, ya know?

I used to have a DVD on barrel making in the old days showing the gunsmith at Colonial Williamsburg Wallace Gussler and his apprentice hand forging a M/L barrel around a mandrel, this process was very time consuming and leaves the barrel with a lot of hammer indentations, It was possible to hammer the barrel roughly octagonal and then dress it up with a file afterwards. Nearly all the original american long rifles had swamped octagonal  barrels. It is probably still possible to get a copy of that DVD somewhere but I sold mine years ago.
As a matter of interest the last time I was at the NMLRA range in Friendship over ten years ago Mr Gussler had a booth there selling M/L stuff, he may still trade there during the spring and fall comps.
Mike.