Unity 3D - Instantiate when last object hit

public class weaponScript : MonoBehaviour
{
        public gameobject projectilePrefab;
        public bool launchable;
        private projectile projectileScript;

        void Update{
        if(launchable){
            GameObject projectileIntantiated = Instantiate(projectilePrefab, firePoint.transform.position, transform.rotation);
            projectileScript = projectileIntantiated.GetComponent<projectile>();
            projectileScript.parentWeapon = this.gameObject.GetComponent<weaponScript>();
            launchable = false;
        }
    }


public class projectile : MonoBehaviour
{
    public weaponScript parentWeapon;

    void OnCollisionEnter2D(Collision2D other){
        parentWeapon.launchable = true;
    }
}