On the area where you can put your main guns (the R-slots) it has a certain number of space. Each ship has a limited space. Each gun requires a certain amount of this space. That's why you won't see a DD with dual 14 inch guns, even if it had enough space displacement.

Gun-space?

I don't have an American crew that can use the Dual 6s, so my only guide is trainworld. And...

Lv 35 x2 6"/ 47 DP cal Mark 16

That's all I know. *Shrug*.

Maybe the ship you're trying to use it on can't fit the 6s?

Edit: Ah, wait, the non-dp guns. Stupid trainworld should have the guns in level order :p. Then ignore what I said about the level thing. Again, what ship are you trying to mount them on?

Uh hailene, you kidding? My gunners are level 34 and I am using Dual 6"/53 Cals on my Atlanta.

Americans don't get the dual 6s 'till 35.

gottcha

you should post this in technical support