I assume its included as no extra fire control tonnage is mentioned in Cstars ship stats.
If I was deckplanning a turret or such armament, it'd be half a dton for the fire control comp and half a dton for Gunner workspace/access space.
This would show up on the deckplan as a square (0.5 dton) with a station inside it, then a square is taken up (the other 0.5dton) as the drawn turret.
So, for the Railgun Barbette, I'd say excluding the ammunition (1 dton), there would be some larger space for the Gunner workspace/access space (with a drawn seat for the gunner) -this seems to be the case as Cstars has a Weapons Control room for the Gunner- and the rest for the fire control, which is taken up either as the drawn railgun or added as a little desk by the chair).
Either way, excluding ammunition, I have around 8 squares to play with on the deckplan for the Weapon Control room, though some of those sqaures may take up a drawn Railgun silhouette on the exterior (added fire control comp space, depending on complexity).
Whats worse is that in Cstars they tend to have circular rooms so there are partial sqaures in these areas - would half squares in a circular room be counted towards full dtons or halfs?