Sorry - maybe I'm late to the game here but you don't need the 1-ton for fire control for barbettes! Only for Turrets and and Bays!
I just flipped through all the book designs and the rules on both the SRDs and the designs themselves. They seem to confirm that only turrets have that volume (1 or 2 tons depending on the turret) and bays (add 1 ton per bay!) - but all barbette entries seem to be at their flat volume with no additions.
I just flipped through all the book designs and the rules on both the SRDs and the designs themselves. They seem to confirm that only turrets have that volume (1 or 2 tons depending on the turret) and bays (add 1 ton per bay!) - but all barbette entries seem to be at their flat volume with no additions.