I did a Snapshot vs A6m2s and barely got 4 kills with the 37mm before I was out of ammo, the 20mm option I landed 9.
Frankly unless you are 400 behind and they are leveled out, you won't get a kill, snapshots are almost impossible due to slow rate of fire.
However in a nice turn fight and you can close to 200 or less - its a single shot to kill.
Honestly I don't fly the 37mm option and probably wouldn't, even the 39Q I won't fly, if anything the P39D with 20mm option just for the kicks of flying it.