If you are into Pirate reenacting and the like, I would suggest you use either Hern's English swivel gun (which can go on a naval carriage) or the US Carronade (which is actually a gunnade, discussed in recent topics). These have much more Pirate-ish lines (are more in the pirate time line) and have more character on a naval carriage in my opinion.
As much as I try to stay period correct with my builds, I am swayed into using steel barrels that are turned on a lathe because the lines and contours are so sharp (which looks good to me). With a replica of a cast iron gun, a Hern barrel is probably closer to the way an original may have looked, but I would still have a turned gun if possible. I think there are some sponsors here who can do lathe turned guns and I know Zulu makes great naval carriages for any barrel.