No, I can't drag a crew member into the dark grey area for the deck guns. Believe me, I tried.
Also, I have tried dragging crew with or without the order to man the guns (the green light) active, still no success.
Also, you can see the "slots" for the repair crew and the Flak. But the deck gun doesn't even have those empty slots anymore. Strange.
Well, after the last patrol I got me my old deck gun (behind the conning tower) back, to see what would happen: still can't drag crew to man it.
