Try something similar to this.
You'll want to use the X coordinate from the camera as the process variable to a PID controller (since this will change with the rotation of the turret). Your setpoint should be the center of the image (where you want the target to appear). This can be found by dividing the X resolution of the image by two. Note that this also assumes that the camera is mounted exactly on the center of the shooter - you might have to tweak the setpoint to make it mesh with your system. You'll need to tune the PID constants in my snippet - since they're the default values. There are other threads around for help on PID gain tuning (just
search for them).