What these other guys said.

FRX
You are lucky that you've discovered so quickly that you've made an error levelling your
gunners. You should dismiss those sailors and re-roll two more sailors who are
10acc/11reload.

Then leave as armament sailors until level 65 at the earliest and lvl 85 if you can be
patient. Your level 40 gunners are worthless because the reload stat is too low and
unclassed armament sailors level up much more quickly than classed gunners.

Please see Longrooster's guide to correct class (stickied in this forum). Waste no more
time with your lvl 40 gunners.

If they're still armament sailors, the dual 3.9" or 5" NLD. Be aware that the limited
space (especially T-slots) on some ships limits your options.

If they're already AA gunners, it's probably worth rolling up a second set to class late.
In the mean time, the dual 3.9" A has a good rate of fire and okay damage (if meh range).
It also has the advantage of being tiny (less space than the dual 4.7"), and an actual
dual purpose gun (consider especially if you're taking a Kongo or Asama to blitz)

If your AA gunners are sufficiently BVE'd to produce useful spread, just go with the 4.7s.
The range is remarkably useful.

The dual 3" A is more of a toy than anything else. But sufficiently pom-pom like to be
amusing at times.

***I've heard about that the 4.7"/ 45 Type 10 is not good to use at low levels.***

They probably meant classing right away and using 4,7'' is not the good way to do.