the key about leveling crew in AA ships its the level of your crew and the level of your ship.
this is the same for all nation...
in US: use 3.70 mk23N (they have no L and D lack of range) this guns while heavy can put a nice punch at a decent range (almost equal to KM lvl 30 AA guns) with the chance of load lots of ammo.
5/38 mk38L have only space in most ships to load 4 or 6 packs of ammo, this is only good for secondary purposes above BB3 and beyond. the N and D options of this guns will loose a lot of range and will not add much of ammo.-
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its not the best nation to do it, but if you go AA with US get a pair of 11/11 gunners and make them rel-rel-rel. dont use AA gunners and lvl up a BO that needs to go to the "brookling" ship tree. as all of the ships (except for NM premod) can use decent AA.
the "other" advantage about US AA is that since you are using normal guns you can load 1 bind of AP to engage small ships that might enter your range.
back to start... to level up sailors:
from lvl 12 to 40: use atlanta of any remod (i like juneau the most) it has very funny firing arcs, but you'll get used to it... besides, its not such a great deal to lvl from 12 to 40...
from lvl 40 to 48/50: when you get that lvl move to cleveland, use it as AA ship, using only T slots (use R slots to lvl up sailors) it has great speed
from lvl 48/50 to 60: the "normal" path its baltimore. use it like u used cleveland, it has nice speed and quite good T slots, the only backdraw its the big hitbox, but you will be behind friendly line so its not such a big issue
- if you have the olives buy a Pensa to lvl up from 48/50 up to 60/65, this ship - as any prem ship - will pay more and if you get tired of doing AA all the time you can play as mini-BB, but for AA purposes dont load R slot guns
if you are good with angle management you will have nothing to envy from KM AA ships (since most players uses Lvl 30 AA guns) but try not to pick for escort the same groups of ships than a MN AA ship does, you will get frustrated.