Xbox Gta iv rapid fire heli

You spawn a heli for example a Annihilator .
Then you attach an invisible Annihilator to your Annihilator.
Lock the doors of this Annihilator and spawn a ped in the drivers seat.
Setup a button press so that when X is pressed (miniguns) task combat to the ped.
The ped will then use the invisible Annihilator's miniguns while you use yours. Essentially 2x the miniguns so 2x the speed.
If you exit the Annihilator then it will delete the invisible Annihilator and the ped.

 

Or you can get the groundZ coords for the pitch of your helicopter and create an explosion effect there............ Easier than creating another object and to set it up as well as somehow make it shoot without a driver.