A few C# problems.

Below are two scripts, one for summoning a projectile (goes on the gun), and the other for collisions which goes on the projectile. The first problem is "Cannot cast from source type to destination type." While i have googled the problem none of the answers explain what is wrong but rather give a specific fix. Full debug.log
InvalidCastException: Cannot cast from source type to destination type. WeaponScript.ShootingFunc () (at Assets/Script/WeaponScript.cs:43) WeaponScript.Update () (at Assets/Script/WeaponScript.cs:23)

The second problem i'm having is that every time i fire the Ammo var is supposed to go down by one, however it does not. Also i have a simple delay between shots and this is also not working, is this due to the invalidCastException?

This is the code for the weapon and projectile instancing

 using UnityEngine;
 using System.Collections;
 
 public class WeaponScript : MonoBehaviour {
     public Rigidbody projectile;
     public int ammoLeft = 0;
     public bool isWeaponCycling;
     public float weaponCycling; 
 
     void Start () {
 
         isWeaponCycling = false;
         weaponCycling = 5.0f;
 
     }
 
     void Update () {
     if (Input.GetButtonDown ("Fire1")) 
         {
             if (isWeaponCycling == false)
             {
             ShootingFunc();
             }else
             {
             Debug.Log("Cannot Fire at this time");
             }
 
         }
         if (isWeaponCycling)
         {
             weaponCycling -= Time.deltaTime;
             if (weaponCycling < 0)
             {
                 isWeaponCycling = false;
                 weaponCycling = 5.0f;
             }
         }
     }
 
     void ShootingFunc ()
     {
         Rigidbody bullet = (Rigidbody)Instantiate(projectile, transform.position, transform.rotation);
         isWeaponCycling = true;
         ammoLeft -= 1;
         Debug.Log ("You have " + ammoLeft + " rounds left");
 
             if (ammoLeft == 0)
             {
             Debug.Log ("Horey shit guys no ammo left");
             }
         }
 }

The other script goes on the actual projectile.

 public class ProjectileScript : MonoBehaviour {
     public int projectileSpeed = 500;
     public float timeTillDeath = 10;
 
 
         void FixedUpdate () {
                 rigidbody.AddForce (Vector3.forward * 10);
         }
 
     void Update () {
         transform.Translate (Vector3.forward * Time.deltaTime * 600);
         timeTillDeath = timeTillDeath - Time.deltaTime;
 
         if (timeTillDeath < 0) 
             {
             Destroy (gameObject);
             }
     }
 
     void OnCollisionEnter(Collision col){
         if (col.gameObject.CompareTag("Enemy")){
             col.gameObject.SendMessage("HealthAdj", -10, SendMessageOptions.DontRequireReceiver);
         }
         Destroy(gameObject); // bullet suicides after hitting anything
     }
 }
 
 

Thanks in advance, this has been bugging me all morning.