Nath, a few questions.
First, did you make sure the other gun turrets are out of ammo when you tried this with the tail gun.
Second, if you didn't, did you know that when you fire the tail gun, either the top turret or ball turret will also fire...no matter what.
Third, did you tap the "F" key on the keyboard.
Fourth, you do know that there are more bullets flying other than tracers (about 1 in 4 or 6 is a tracer...so there are dozens of bullets you cannot see in the air).
I did an experiment by shooting the ball turret at the ground. I tapped the fire button and I saw nothing wrong.
Hans.