Even if the physics behavior is a little tricky , it's potential is huge !
In your case , You shoudn't use the physics behavior with the turret ( You don't want the turret to be affected by the gravity ) , the bullet is the one that is affected by gravity .If you are wondering how to make it look real , Just copy the ghost shooter example , Spawn a bullet and give it an impulse with angle as the turret's angle !
That should do the job !
Cheers