Currently, all 3 aircraft take no damage from AR rounds or even .338 machinegun rounds. 150 rounds of 7.62 fired from the littlebirds miniguns only brought the neophrons health down to orange. In real life, most combat aircraft have very little armour and major components are only covered with by the composite shell of the aircraft, thus even small arms should be able to damage aircraft.
Now, obviously, small arms dont possess the range to make an effective AA weapon but a plane that is flying low and slow should be vulnerable to concentrated fire from an infantry squad. The A-10 being a possible exception as it has some parts armoured.