It is nearly impossible to not acc and reload cap USN gunners.
+9s will cap for US with 100ish vets (Which is easy to achieve, even for free).
The best classing for USN is straight reload, especially if you have a boost and 100+ vets early.
If you don't expect to be able to boost your gunners or get 100 vets, acc/rld/rld is the best classing for 100% Free gunners. It gives you the early accuracy boost to counter the spread on the early USN BBs, with minimal sacrifice in total reload ability down the line.