Make the turret automatically rotate to look at where the crosshair is aimed at.

I'm not exactly sure what you want from your description, but here are two scripts that combined will have a gun on a turret pointing at a specified game object. As the game object moves, it will track the object. Tracking speed is controlled by the maxDegreesPerSecond parameter. For smaller values and/or larger target movements, the gun will lag behind the target. The gun must be a child object of the turret, and the gun barrel must point forward (i.e. down the Z axis). In the inspector, drag the target object to the GoTarget parameter in both scripts.

--- Turret script ---

 var goTarget : GameObject;
 var maxDegreesPerSecond : float = 30.0;
 private var qTo : Quaternion;
 
 function Start () {
     qTo = goTarget.transform.localRotation;
 }
 
 function Update () {
     var v3T = goTarget.transform.position - transform.position;
     v3T.y = transform.position.y;
     qTo = Quaternion.LookRotation(v3T, Vector3.up);        
     transform.rotation = Quaternion.RotateTowards(transform.rotation, qTo, maxDegreesPerSecond * Time.deltaTime);
 }

--- Gun Script ---

 var goTarget : GameObject;
 var maxDegreesPerSecond : float = 30.0;
 private var qTo : Quaternion;
 
 function Start () {
     qTo = goTarget.transform.localRotation;
 }
 
 function Update () {
     var v3T = goTarget.transform.position - transform.position;
     var v3Aim : Vector3;
     v3Aim.x = 0.0;
     v3Aim.y = v3T.y;
     v3T.y = 0.0;
     v3Aim.z = v3T.magnitude;
     qTo = Quaternion.LookRotation(v3Aim, Vector3.up);
     transform.localRotation = Quaternion.RotateTowards(transform.localRotation, qTo, maxDegreesPerSecond * Time.deltaTime);
 }

Here's some C# code I use for spaceship turrets. I feel this could be simplified a bit but I was never able to get it to work without saving the local rotation and reapplying them to the axes I don't want to rotate. Works no matter how the vehicle is oriented.

The barrels need to be a child of the rotating turret base. The script is a component of the turret base. This code is in the update function (I declare all the variables beforehand to reduce garbage collection).

The turret will traverse on the local Y-axis, and the barrels will pitch on the local X-axis.

     //rotate turret
     tmpRotation = transform.localRotation; //preserves the local rotation since we only want to rotate on local Y axis
     leadTargetPosition = FirstOrderIntercept(transform.position,Vector3.zero,laserParticleLeft.particleEmitter.localVelocity.z,target.transform.position,target.rigidbody.velocity);
     targetPointTurret = (leadTargetPosition - transform.position).normalized; //get normalized vector toward target
     targetRotationTurret = Quaternion.LookRotation (targetPointTurret, parent.transform.up); //get a rotation for the turret that looks toward the target
     transform.rotation = Quaternion.Slerp(transform.rotation, targetRotationTurret, Time.deltaTime * turnSpeed); //gradually turn towards the target at the specified turnSpeed
     transform.localRotation = Quaternion.Euler( tmpRotation.eulerAngles.x, transform.localRotation.eulerAngles.y, tmpRotation.eulerAngles.z); //reset x and z rotations and only rotates the y on its local axis
 
     //rotate barrels
     tmpRotation = barrels.transform.localRotation; //preserves the local rotation since we only want to rotate on local x axis
     targetPointBarrels = (leadTargetPosition - barrels.transform.position).normalized; //get a normalized vector towards the target
     targetRotationBarrels = Quaternion.LookRotation (targetPointBarrels, parent.transform.up); //get a rotation that looks at the target
     barrels.transform.rotation = Quaternion.Slerp(barrels.transform.rotation, targetRotationBarrels, Time.deltaTime * turnSpeed); //gradually turn towards the target as the specified turnSpeed
     barrels.transform.localRotation = Quaternion.Euler( barrels.transform.localRotation.eulerAngles.x, tmpRotation.eulerAngles.y, tmpRotation.eulerAngles.z); //reset y and z rotations and only rotates the x on its local axis
 

Thanks DarthDisembowel edited some word:D

Quaternion tmpRotation = turretGO.transform.localRotation;

     Vector3 targetPointTurret = (lookAtTarget.transform.position - turretGO.transform.position).normalized;

     Quaternion targetRotationTurret = Quaternion.LookRotation(targetPointTurret, turretGO.transform.up);

     turretGO.transform.rotation = Quaternion.Slerp(transform.rotation, targetRotationTurret, 1f);

     //replace 1f = Time.deltaTime * turnSpeed if u want a delay time

     turretGO.transform.localRotation = Quaternion.Euler(tmpRotation.eulerAngles.x, turretGO.transform.localRotation.eulerAngles.y, tmpRotation.eulerAngles.z);

     /////////////////////////////////////////

     tmpRotation = gunGO.transform.localRotation;

     Vector3 targetPointBarrels = (lookAtTarget.transform.position - gunGO.transform.position).normalized;

     Quaternion targetRotationBarrels = Quaternion.LookRotation(targetPointBarrels, transform.up);

     gunGO.transform.rotation = Quaternion.Slerp(gunGO.transform.rotation, targetRotationBarrels, 1f);

     //replace 1f = Time.deltaTime * turnSpeed if u want a delay time

     gunGO.transform.localRotation = Quaternion.Euler(gunGO.transform.localRotation.eulerAngles.x, tmpRotation.eulerAngles.y, tmpRotation.eulerAngles.z);