Having had the opportunity to look at, and take apart quite a few of these great old guns, I would think the barrel finish would have been done with something like a fine flat wet stone, run lengthwise down the barrel.
It seems though, on earlier rifles, this was only done on the visible flats, and the three bottom flats finished with a grinding wheel leaving much more course grinding marks perpendicular with the bore. As a side note, the barrel inlet in the stock was never finished to anywhere near the degree as the exterior of the stock. No sanding, scraping or any oil finish.
I'll try to get a few pictures this afternoon.
John
I should add that I'm talking flint period, and that by the percussion period a lot had changed.