the lvl 25, 25* aa guns, gives great range for scout sniping :)
at least this is what i use on my alsace :P

3.5"A if you want to shoot planes farther from you

5.1"A dual if you want to shoot down squadron of planes in 1 shot

you are better off with the longer range tbh

RM is mostly about rushing so take the long range guns.
Tge GA is 25 i think