I remember making a Displacer Ball shooting Apache once.Displacer ball?
Did you change it's health as well or just the ammo?It'll probably have more health than the normal monster_turret.
You might want to slow down the rate of fire so that it won't create a dead-zone where ever it fires... or I'll reduce the rocket damage and keep it unchanged.
The turret's not quite complete. It still needs tweaking in many more areas..