I have to recall this from memory since it has been a while since I played this. :)
It is possible to shoot down air targets with the turretgun, but I think the developers decreased the effectiveness of it so not to make it too easy, so other weapons are better for taking down airtargets. The most effective way to shoot down air targets with the turretgun was to turn off the WSO, which makes the primary advantage of the turretgun pointless, if you do use the WSO make sure the air target is within the firing arc of the gun (it can't fire upward), also the closer the target is the better chance of a hit.
By the way, armored targets like other Hinds or Apaches cannot be damaged by the Hinds turretgun.