Unity Can't get bullet to fire the right direction

I'm trying to get my tank to shoot in the direction the mouse is facing. Currently the tank can rotate to look at the mouse, but I have no idea how to make the bullet go the right direction!

Currently the turret on the top does rotate correctly, I'm not sure if the rotation on that object can be used for the direction?

It is iin unity 3d but it is really 2.5d angled topdown.

public class tankScript : MonoBehaviour
{
    public float rotateSpeed = 90;
    public float speed = 5.0f;
    public float fireInterval = 0.5f;
    public float bulletSpeed = 20;
    public Transform spawnPoint;
    public GameObject bulletObject;
    float nextFire;

    Camera cam;
    Collider planecollider;
    RaycastHit hit;
    Ray ray;
    Vector3 mouseDirection;

    void Start()
    {
        nextFire = Time.time + fireInterval;

        cam = GameObject.Find("Main Camera").GetComponent<Camera>();
        planecollider = GameObject.Find("Plane").GetComponent<Collider>();
    }

    void Update()
    {
        var transAmount = speed * Time.deltaTime;
        var rotateAmount = rotateSpeed * Time.deltaTime;

        if (Input.GetKey("w"))
        {
            transform.Translate(0, 0, transAmount);
        }
        if (Input.GetKey("s"))
        {
            transform.Translate(0, 0, -transAmount);
        }
        if (Input.GetKey("a"))
        {
            transform.Rotate(0, -rotateAmount, 0);
        }
        if (Input.GetKey("d"))
        {
            transform.Rotate(0, rotateAmount, 0);
        }
        if (Input.GetButtonDown("Fire1") && Time.time > nextFire)
        {
            nextFire = Time.time + fireInterval;
            fire();
        }
    }

    void fire()
    {
        ray = cam.ScreenPointToRay(Input.mousePosition);
        if (Physics.Raycast(ray, out hit))
        {
            if (hit.collider == planecollider)
            {
                mouseDirection = new Vector3(hit.point.x, 0.25f, hit.point.z);
                float step = bulletSpeed * Time.deltaTime;
                var bullet = Instantiate(bulletObject, spawnPoint.position, spawnPoint.rotation);
                bullet.GetComponent<Rigidbody>().transform.localPosition = Vector3.MoveTowards(transform.localPosition, mouseDirection, step);

            }
        }
    }

The key issue here is that you likely want to set the velocity of the rigidbody rather than moving it by setting the localPosition. Otherwise you lose all the benefits of the physics system, and won't be able to detect collisions correctly.

What value should you set it to, though? Well, velocity consists of two components:

• Direction: This should be the difference between the mouse hit point and the spawn point.
• Speed (magnitude): This should be bulletSpeed.

Let's change your script to see what using velocity instead of manually moving the bullet would look like:

<!-- language: lang-cs -->

void fire()
{
    ray = cam.ScreenPointToRay(Input.mousePosition);
    if (Physics.Raycast(ray, out hit))
    {
        if (hit.collider == planecollider)
        {
            mouseDirection = new Vector3(hit.point.x, 0.25f, hit.point.z);

            // This vector is the direction vector we want to fire along
            Vector3 firingDirection = mouseDirection - spawnPoint.position;

            var bullet = Instantiate(bulletObject, spawnPoint.position, spawnPoint.rotation);

            // Now, we set the velocity to firingDirection, but scaled to bulletSpeed 
            bullet.GetComponent<Rigidbody>().velocity = firingDirection.normalized * bulletSpeed;

        }
    }
}