I think I've got it. A shame I have to make the rotational bit (as it's not the whole gun, it's just two little bits that are joined on to the ship) the weapon model, but it'll work. Now I just have to find a way to lock primary armament selection while still allowing secondary selection in the briefing.
you can prevent other types of weapons from being equipped to a certain bank (using $allowed pbanks: and $default pbanks:), but i dont think you can enforce the bank so that there is always a weapon there. might launch without one for example. id really like a feature that treats a weapon as part of the ship, where its always equipped (and possibly for ballistic weapons allow partial weapon loads for primary banks).
i brought up another idea a few days ago in another thread, to do a banktypes system. each bank is assigned a type, each type containing a collection of bank settings. like whether to show weapon models in the loadout screen, allow weapon to be changed, ammo characteristics, energy usage, autoaim features (instead of being a per ship or per weapon thing, would be awesome at the per bank level, like having a gimballed gun mount that can accommodate any weapon). possibly also change the way primary and secondary banking is handled, for example treat a secondary bank like a primary bank and vise versa. you could possibly also configure linking and selecting characteristics for various banks. just an idea, im considering it for my own engine, but i dont see why it wont work in fs.