class object is not destroying

 public class C_Shell : MonoBehaviour
 {
     public GameObject shellObject;
     public bool countdown;
     public float deleteTime;
     public Vector3 velocity;
     public Vector3 position;
 
     public C_Shell(GameObject _shellObject, float _deleteTime, bool _countdown, Vector3 _position, Vector3 _velocity)
     {
         this.shellObject = _shellObject;
         this.countdown = _countdown;
         this.deleteTime = _deleteTime;
         this.position = _position;
         this.velocity = _velocity;
     }
 
     public void SelfDestruct()
     {
         Destroy(this, this.deleteTime);
     }
 
     public void UpdateObject()
     {
         this.shellObject.transform.position = this.position;
     }
 }
 
 public class Shell_Manager : MonoBehaviour
 {
     public static Shell_Manager instance;
 
     public const int maxShells = 128;
 
     public List<C_Shell> shells;
 
     public float deleteTime = 3.0f;
 
     private void Awake()
     {
         if (instance == null)
             instance = this;
     }
 
     private void Start()
     {
     }
 
     private void Update()
     {
     }
 
     private void FixedUpdate()
     {
         UpdateShells();
     }
 
     private void UpdateShells()
     {
         int index = 0;
         foreach (C_Shell shell in shells)
         {
             shell.SelfDestruct();
 
             shell.countdown = true;
             shell.position += shell.velocity * Time.fixedDeltaTime;
             shell.position += Physics.gravity * Time.fixedDeltaTime;
 
             index++;
         }
 
 
     }
 
     public void FireShell(ShellFire _shell)
     {
         Vector3 initialPosition = _shell.initialPosition;
         Quaternion initialAngle = _shell.initialAngle;
 
         switch (_shell.eShellType)
         {
             case EShellType.AP:
                 {
                     Fire_AP(initialPosition, initialAngle);
                     break;
                 }
             case EShellType.HE:
                 {
                     Fire_HE(initialPosition, initialAngle);
                     break;
                 }
         }
     }
 
     void Fire_AP(Vector3 _initialPosition, Quaternion _initialAngle)
     {
         Object prefab = AssetDatabase.LoadAssetAtPath("Assets/Resources/Prefabs/Shell.prefab", typeof(GameObject));
 
         shells.Add(new C_Shell(Instantiate(prefab, _initialPosition, _initialAngle) as GameObject, deleteTime, false, _initialPosition, Vector3.forward * C_AP.constants.force));
     }
 
     void Fire_HE(Vector3 _initialPosition, Quaternion _initialAngle)
     {
         Object prefab = AssetDatabase.LoadAssetAtPath("Assets/Resources/Prefabs/Shell.prefab", typeof(GameObject));
 
         shells.Add(new C_Shell(Instantiate(prefab, _initialPosition, _initialAngle) as GameObject, deleteTime, false, _initialPosition, Vector3.forward * C_HE.constants.force));
     }
 }

I dont know why is C_Shell object not being destroyed