If we want to imagine what turrets, barbettes and bays look like wouldn't we be better looking at warplanes? :CoW:
The best bet is to look at what's already been defined in canon. In canon (to wit, CT:S7 T&G, & TNE:FF&S), as in WWII, barbettes are large turrets. (In TNE, they aren't limited to just PAW.)
TNE:FF&S specifies they are a can with a turret head sticking up just a bit, comprising 70kL of weapon. The Turret is a smaller can with a smaller turret head.
CT shows us a 1Td (or just shy) can and a small extension of turret head past the hull.
Based upon the CT difficulties of field replace, and the TNE ones, they seem to be essentially the same kind of construction.
Note that Bays are just much bigger cans, based upon FF&S; in CT, AHL shows us that they're great big turrets.