Unity3D: Use raycasts to create projectiles - Source: http://answers.unity3d.com/questions/4867/using-raycast-to-shoot

RaycastShooting.js

var  par : ParticleEmitter;
var damage = 2;

function Update () {

var hit : RaycastHit;
// Use Screen.height because many functions (like this one) start in the bottom left of the screen, while MousePosition starts in the top left
var ray : Ray = Camera.main.ScreenPointToRay(Vector3(Input.MousePosition.x, Screen.height - Input.MousePosition.y,0));

    if (Input.GetMouseButtonDown(0)) {
        if (Physics.Raycast (ray, hit, 100, kDefaultRaycastLayers)) {

            Instantiate(par, hit.point, Quaternion.LookRotation(hit.normal));

            var otherObj : GameObject = hit.collider.gameObject;

            if (otherObj.tag == "Enemy") {
                otherObj.GetComponent(typeof(HealthAI)).CanDie(damage);
            }
        }
    }
}


var aihitPoints : float = 10;

function CanDie ( damage : float) {
    aihitPoints -= damage;
    if(aihitPoints <= 0 ) {
      Destroy(gameObject);
    }
}