I think that is like this, never tried but you can, in _harrier.gsc search for weapon, and you will see this 2, one is the harrier missiles and the other the harrier cannon.
Replace those with the guns you want,Code:self setVehWeapon( "harrier_20mm_mp" ); //cannon self setVehWeapon( "harrier_FFAR_mp" ); //missiles