Here is the rule when it used to make sense and didn't have page after page of discussion:
Weapons: A small craft may mount the equivalent of one turret. In actuality, the mountings are probably rigid, and no actual turret is present. All computations, however, may assume that the craft carries one turret. Weight, tech level, cost, and energy point restrictions must be observed. The pilot is assumed to be the gunner for one type of weapon on the craft. If additional types are mounted (a craft could conceivably have three different types of weapons), a gunner is required for each additional weapon. Exception: no additional gunner is required for sandcasters.
This is of course from the 81 edition of CT HG.
Small craft can mount the equivalent of one turret - so a triple mount (or quad now that T5 has green lit this) 1t turret, or a 2t energy weapon turret, or even a 3t PA turret, but no barbette.
The problems start to arise when you try and mix the above weapon mounts. Can you stick eight beam lasers in a 2t turret? Or how about twenty beam lasers in a 5t barbette?
Then the Solomani Alien book confused things even further with actual fixed mounts:
Fixed Weapons Mounts: Solomani ships may have fixed weapons mounts (as opposed to turret mounted weapons).
Fixed weapons mounts allow up to two weapons to be attached per each hardpoint on the ship and do not require fire control tonnage or a turret. Weapons in fixed weapons mounts are operated by a gunner on the bridge and are subject to an attacker's DM - 2 in space combat.
A ship is allowed fixed weapons mounts equal to the model number of the computer installed on the ship (Model/2 computer allows two hardpoints to have fixed weapons mounts).
Still limited by hardpoint number based on ship size.
No turret or fire control - so therefore a book entry of 0 for mass displacement. Your fixed mount weapons cost no tonnage.
So a starship fixed mount may be two lasers, two fusion guns or even two particle accelerators.
But a small craft 'fixed mount' is not actually a fixed mount, it is a rigid mount. It comes with a displacement cost of 1 ton of fire control minimum all the way to 3t if it is a PA.
The next issue is the difference between the LBB2 fire control and the LBB5 turret - in LBB2 you paid for the fire control as a tonnage cost and the turret free, but in HG they changed it to a tonnage cost for the turret with no fire control cost.
So what exactly is the difference between a 1t basic turret and a 2t energy weapon turret? How many beam lasers can a 5 t barbette hold?