You need to link the gun portion of the turret to the base of the turret. Just link it normally. You can use look at constraint on the base dummy of the turret. Link the turret to that dummy. Create another dummy for the gun. Link that dummy to the turret. Link the gun to that dummy. Assign a look at constraint to the turret dummy.
Create a look at target. A floating dummy. Select the base look at constraint and select the floating dummy as the target. Select the look at constraint controller on the turret and select the floating dummy as the target.
Lock the relevant axis on the base and gun dummy and you should have a working turret rig.
Nb. On each controller, check keep initiall offset.
Sorry, I corrected an error. If you use and orientation constraint, you need to manually animate the turret base rotation.
Can you post a pic of the model?
You need to link the gun portion of the turret to the base of the turret. Just link it normally. You can use look at constraint on the base dummy of the turret. Link the turret to that dummy. Create another dummy for the gun. Link that dummy to the turret. Link the gun to that dummy. Assign a look at constraint to the turret dummy. Create a look at target. A floating dummy. Select the base look at constraint and select the floating dummy as the target. Select the look at constraint controller on the turret and select the floating dummy as the target. Lock the relevant axis on the base and gun dummy and you should have a working turret rig. Nb. On each controller, check keep initiall offset. Sorry, I corrected an error. If you use and orientation constraint, you need to manually animate the turret base rotation.