Now for tactics with the machine gun. For starters, make sure that when you are bored while your pilot is doing tricks through the air while you cant see anything, DO NOT IDLY FIRE OFF YOUR MG FOR NO PARTICULAR REASON!!! Why is this so important? Because the machine gun bullets are tracer rounds (thanks GareBizzle) that travel a very long way, and by randomly firing you will be endangering you and your pilot by letting others know where you are in the sky. I have snuck up on many an unsuspecting foe while their gunner is idly firing, and have that way gotten a double kill very quickly and easily. Just remember that when you have that tasty looking cannon in front of you, do not abuse it, or it may very well cost you an extra death.
In order to properly use the machine gun, you must also realize that it is like any other gun in BF2142; it has its own bullet speed. This means that you CANNOT randomly fire at objects and expect your bullets to hit just because youre aiming at it, you must aim ahead. Anyone who excelled as a sniper in BF2 should understand what I mean especially well, as you sometimes have to aim well ahead with the sniper rifle as well. Yet again, it takes practice to know just the right way to aim with this gun, and the single player computers come in especially handy for this, as you can switch to the gunner position without fear of crashing.
Lastly, vehicle tactics with the machine gun. The machine gun CANNOT damage enemy APCs, Tanks, Walkers, Titans (including the exterior guns), or Goliaths. The machine gun can efficiently get kills by shooting any other vehicle or turret in the game, either via splash damage (hitting the outside of a turret and killing the person inside through that), or via direct hits (shooting cars, gunships, TRANSPORTS). Transport ships are probably the most fun to shoot this way, because if you see a person hanging out the side of these, feel free to pick them off with your machine gun! They will die inside the vehicle, without it even exploding. After that you can give it a nice quick TV Missile to finish off the rest of it. If Ive said it once Ive said it a thousand times, it takes time and practice to know which vehicles are affected in which ways by the machine gun.