Some of these guns were made to be used in the "crows nest" for shooting down on the enemy's quarterdeck. The swivel mount stayed on the nest rail and the gun was hoisted up before battle. The enemy had guys in their nest, and they were shooting at you. The gun was attached to the swivel so it wouldn't drop to the deck when the guy shooting it was killed. Generally, there was a crew to load and fire it. the gunner, often a midshipman, a wiper/rammer, and a powder boy.
A ship of the line would have these stations built all over the masts. Because life on a British Naval vessel was a brutal experience, a "pressed man" didn't get to shoot it...he might just take out the officer who had had him flogged. Nice gun!