Unity Cannon fire

using UnityEngine;
using System.Collections;

public class Shoot : MonoBehaviour
{

    public Transform _thePrefab;
    public int forceSpeed = 10000;
    public int fireMinDelay_ms = 500;
    public int fireMaxDelay_ms = 1000;
    private float nextFire = 0.0f;
    private float delay;

    void Start()
    {
    }

    void CannonFire (Transform fire_position)
    {
        Transform prefabInstance = (Transform) Instantiate(_thePrefab, fire_position.position,Quaternion.identity);

        prefabInstance.rigidbody.AddForce(fire_position.forward * forceSpeed);
    }

    // Update is called once per frame
    void Update ()
    {

        if (nextFire != 0.0f && Time.time > nextFire)
        {
            nextFire = 0.0f;

            var fire_position = transform.Find("SpawnPoint");
            CannonFire(fire_position.transform);

        }

        if (Input.GetMouseButtonDown(0) && nextFire == 0.0f)
        {
            delay = Random.Range(fireMinDelay_ms,fireMaxDelay_ms)/1000.0f;
            nextFire = Time.time + delay;
        }
    }
}