Interesting.
Using a 5"-er in early boats would certainly make things more interesting.
I might just take a whack at the 3d model....I still got a couple of hours before bed
EDIT:
I've done some fiddiling around with the 4" gun, editing it's size, bore, seat removal, I modified the handle that opens the breech, other little details, I'll upload a pic here in a couple
ANOTHER EDIT:
Kinda what ur looking for Ducimus?

comparison shot next to a 4" barrel and body (4" is selected in red)