You could replace the "Browning" on the M2mgun with a ZSUcannon.
The ground MG's - be it soldiers or the static mGun - won't fire at
heavy armored aircraft; but they'll fire at lighter birds like the Cessna plane.
Depending on the (addon?) you're using, try this ( INIT field of an M2mg ) :
This RemoveWeapon "Browning"; This AddMagazine "ZSUcannon"; This AddWeapon "ZSUcannon"
Then I promise ya that he WILL fire at ANY hostile bird that does a fly-by.