Código del cañón: https://youtu.be/RCB41eCLRKA

Bullet.cs

using UnityEngine;
using System.Collections;

public class Bullet : MonoBehaviour {
    public float Speed = 1;
    public float LifeTime;

    private float _age;

    void Start () {
        transform.parent = null;
    }
	
    void Update () {
        _age += Time.deltaTime;
        this.transform.Translate(new Vector3(0, 0, Speed) * Time.deltaTime);

        if (_age > LifeTime) {
            Destroy(this.gameObject);
        }

        // transform.forward
        // transform.left 
        // this.transform.position = this.transform.position + new Vector3(0, 0, Speed) * Time.deltaTime;
    }
}

Cannon.cs

using UnityEngine;
using System.Collections;

public class Shooter : MonoBehaviour {
    public float CooldownTime = 0.5f;
    public GameObject BulletPrototype;

    private float _timeSinceLastShot = 0;

    void Start () {
    }

    void Update () {
        _timeSinceLastShot += Time.deltaTime;

        if (_timeSinceLastShot > CooldownTime) {
            _timeSinceLastShot = 0;
            GameObject clon = Instantiate(BulletPrototype);
            clon.SetActive(true);
            clon.transform.position = BulletPrototype.transform.position;
            clon.transform.rotation = BulletPrototype.transform.rotation;
        }
    }
}