To expand on the guns a bit, there is a good possibility that these could have been Venetian guns which were breach-loaders. These guns were firing stone balls around 25-30mm in diameter.
This suited the metallurgy and foundry-making of the time, too. It's much easier to hammer a barrel around a mandrel than to bore a casting or forging out. Using a mandrel also allows forging reinforcing bands around the barrel, too. Then the breach can be formed from a stout casting, but one short enough to sand cast the chamber in the mould, rather than having to machine out the chamber.
This era was also one with charges that ran 1-2 x shot weight due to the quality of the powder. And to allow for the loose "windage" required firing stone shot.
The breach would be a closed tube of heavy dimension, with a serious flange fitting under the last barrel band. the breach would be loaded, then set in place in the barrel with wedges driven between the aft end of the breach and its gun mount.
Compared to an 18th century 3 or 4 pounder cannon, it would be a paltry and weak thing. But, compared to the other sea-going cannons, of its time, more than enough. Particularly since such guns were for use against raiders and pirates. "Regular" navies closed to contact and used the force of armed mean to board enemies.
As black powder became more chemically consistent, and the purity of the ingredients stabilized, and the infrastructure to cast iron and specialty shot became more common, as simple slip joint became impractical to use in ship-board cannon. Land-based practice also changed. Cannon were no longer semi-fixed siege weapons, but mobile items that were expected to move on recoil. Tactically, this offered an advantage that our learned professor alluded to--a recoiling muzzle-loader could be reloaded from within the (relative) safety of the hull.