Make a new sailor that (that you buy with olives) that has auto control of secondary guns on BB's.  Then you can set the range inwhich they would fire on an incoming ship.

Example.  Buy two new sailors that are "auto-gunners".  lvl them up and then when you place them on a bb1-6 as your T-slot gunners (except AA or HH) they man the guns against any ship that is within their range. 

The benefit--or attraction to purchasing these--would be that they allow bb's to keep attacking on the battle line with main guns while the smaller guns assist against close ships or the little ships like dd's or cl's that race across  (or a surfaced submarine).

-or-

Of course you could also make it an option on certain guns that could be equipped.  (but i threw out new sailor to provoke interest in actually making it happen.)