I can't assure anyone that the barrel is steel, but I should explain the reason I guessed steel. If the maker was going to make a cast-iron cannon, he'd almost certainly have a pattern made which had trunnions integral to it, and a more "conformal" profile than the trunnion band shape.
The separate trunnion band was one of the better ways to put trunnions on a cylinder that began life without trunnions. Another option is wrought iron for one or both parts, but I'm highly doubtful that either part is cast-iron due to shape and 2-piece construction.