I recall seeing them somewhere, but can't remember exactly where. I will dig and see what I can find.
Open mount or turret? I've ordered white metal cast open mounts, only about $3.60 but not great quality. That link is somewhere on my dead hard drive but I might be able to find it again. I have a simple turret made up, we can probably work something out if you can't find anything else.
I need a pair of the single turrets for my target/convoy ship. I'm doing a Utah AG-16. I think I have the link for the open mounts. I was going to just cobble those together. They don't have to be "fine scale" I am half consdering getting some little sheds or barns of the appropriate scale instead of the turrets.
That's quite a target ship! The turrets are not that hard to make, they're pretty small. Go to www.hnsa.org and look up the ordinance book - it has dimensioned drawings of the turrets.
Open mount 5" guns are available in 1/144 as detail parts for plastic subs. Try www.freetimehobbies.com Ron Hunt