Hi Mark,
I wish i had myself a swivel/deck gun to picture and post it here
.
Attached are perspective pictures of the replica mounted on a 1500's discoveries nau (ship), that was built in a ship yard close from my place.
By the time of Portuguese King Dom Manuel I, a great impeller of naval weaponry, these guns were called berços (cradles), also it appears that this term originated elsewhere.
Also attached is a real one, possibly from the XII century, kept at the Oporto military museum.
Fernando
.