"There is something to it I don't grasp. That Nat Geo pic is interesting, with a linear cut in the barrel and multpile reinforcements there."
I would agree that the drawing, by showing only a single linear cut in the barrel, is unusual when it comes to portraying most commonly encountered "hoop & stave" constructed wrought iron barrels.
I think the photo below explains more clearly how the bore is formed, with staves, & the outer barrel layer with continuous hoops. Rather like a wooden barrel made by a Cooper - hence how barrel got its name. This barrel, being quite small, uses only four staves for the bore and, because the piece has trunnions, the hoops are not raised to "grip" the carriage.
Incidentally this one is also mounted on a replica carriage from the "Mary Rose".
Adrian