diverging from the 2 frame conversion for a moment
they did it with a special swage block on an anvil and a lot of file work by the over worked apprentice
seeing a barrel made the original way from a flat wrought iron skelp forge welded around a mandrel will increase your admiration
it is a little hard to see- but if you visualize a line from breech to muzzle - you will see a smaller waist mid way between the sights
it is a little exaggerated in the drawings
http://buckeyebarrels.com/bspecs/lancaster.htm