Setting a Target for projectile.

i am currently having the following problem:

when i set the target for my projectile and ray cast on a non viable target while the projectile is in mid air it produces an odd behavior of the projectile ping ponging between the invalid target and going forward in it's direction. i suspect the problem is related to the ray casting in the update function. but i don't know a better alternative. so if some one could kindly explain to a beginner what is exactly is the cause and how i can overcome it because i am facing similar problems across my game. aka spammed behavior. my goal is to create a delegate at the end that invokes the attacking and pass it to my gui system. thank you very much for your help. A)Tank Script

 public GameObject projectile,barrelEnd;

 public GameObject Target {
     get {
         return target;
     }
     set {
         target = value;

         if(value!=null&&value.
            GetComponent<ObjectLogic>()
            .myType!=UnitType.Player) {

             StopCoroutine("AttackTarget");
             StartCoroutine("AttackTarget");
         }else {
             target = null;
             Debug.Log("Invalid");
         }
     }
 }

 private RaycastHit hit;
 private GameObject target;

 private void Start() {
     target = null;
 }

 private void Update() {
     Behaviour ();
 }
 

 public void Behaviour() {
     if (Input.GetButtonDown ("Fire1")) {
         RayCast();
     }
 }

 public void RayCast() {
     var ray = Camera.main.
         ScreenPointToRay (Input.mousePosition);

     if (Physics.Raycast (ray, out hit)) {
         Target = hit.transform.gameObject;
     }
 }