As a rule of thumb, is a vessel capable of carrying an equal weight of weapons, but distributed differently?
For example, if a vessel has a 5" single mount weighing in at 20 tons, could it mount a dual 3" weighing the same? Or even 2-3" sperate guns weighing 10 tons each?
I'm sure there are other variables, such as how "high" the weapon is as far as effecting center of gravity, proper spacing to place the weapons, etc. etc. And I would think ammo is an issue as well, but from what I know that was stored lower in the vessel on many weapons, even though they have some stored topside.
I'd guess you would take the weight of the torpedoes if it has any as well to figure topside weapon weight? And their launchers, which I would think would be rather light.