Rotation stops updating when gameobject is moving..?

Hello there. I am trying to make a little hovertank game where I want the tank body to keep a fixed rotation while moving and the gun to rotate. The script below works fine when the tank is stationary, but the gun cant rotate once the tank is moving. Some help would be much appreciated.

public float speed;

Rigidbody playerRigidbody;
Vector3 movement;
int floorMask;
float camRayLength = 100f;
GameObject gun;
Rigidbody gunRigidbody;

void Awake ()
{
    playerRigidbody = GetComponent<Rigidbody> ();
    gun = GameObject.Find ("Gun");
    gunRigidbody = gun.GetComponent<Rigidbody> ();
    floorMask = LayerMask.GetMask ("Floor");
}

void FixedUpdate ()
{
    float h = Input.GetAxisRaw ("Horizontal");
    float v = Input.GetAxisRaw ("Vertical");

    GunTurning ();

    Move (h, v);
}


void Move (float h, float v)
{
    movement.Set (h, 0f, v);
    movement = movement.normalized * speed * Time.deltaTime;
    playerRigidbody.MovePosition (playerRigidbody.position + movement);
}

void GunTurning ()
{
    Ray camRay = Camera.main.ScreenPointToRay (Input.mousePosition);
    RaycastHit floorHit;
    if (Physics.Raycast (camRay, out floorHit, camRayLength, floorMask))
    {
        Vector3 playerToMouse = floorHit.point - transform.position;
        playerToMouse.y = 0f;
        Quaternion newRotation = Quaternion.LookRotation (playerToMouse);
        gunRigidbody.MoveRotation (newRotation);
    }
}

}

I have the exact same problem, I just don’t know why…