Bullet.cs

using UnityEngine;

public class Bullet : MonoBehaviour
{
    float force = 1.0f;
    float currentTime = 0.0f;
    float sleeptime = 1.0f;

    public void Initialize(Transform transform)
    {
        gameObject.SetActive(true);
        currentTime = 0.0f;
        this.transform.position = transform.position;
        gameObject.GetComponent<Rigidbody>().velocity = Vector3.zero;
        gameObject.GetComponent<Rigidbody>().AddForce(transform.forward * force);
    }

    public void Update()
    {
        if (currentTime >= sleeptime)
        {
            Sleep();
        }
        currentTime += Time.deltaTime;
    }

    public void Sleep()
    {
        gameObject.SetActive(false);
    }

}

Gun.cs

using UnityEngine;

public class Gun : MonoBehaviour
{
    //PoolGeneratorをつけた空のオブジェクト
    [SerializeField]
    PoolGenerator poolGenerator;

    void Update()
    {
        if (Input.GetKeyDown(KeyCode.Z))
        {
            GameObject bullet = poolGenerator.GetInactiveObject();
            bullet.GetComponent<Bullet>().Initialize(transform);
        }
    }
}

PoolGenerator.cs

using System.Collections.Generic;
using UnityEngine;

//空のオブジェクトにつけておく
public class PoolGenerator : MonoBehaviour
{
    //Poolしたいオブジェクト
    [SerializeField]
    GameObject prefab;
    //生成したい数
    [SerializeField]
    int num;
    List<Bullet> list = new List<Bullet>();

    void Start()
    {
        Bullet obj;
        
        for (int i = 0; i < num; i++)
        {
            obj = Instantiate(prefab).GetComponent<Bullet>();
            obj.transform.parent = transform;
            obj.gameObject.SetActive(false);
            list.Add(obj);
        }
    }
    
    public GameObject GetInactiveObject()
    {
        for (int i = 0; i < list.Count; i++)
        {
            if (!list[i].gameObject.activeSelf)
            {
                return list[i].gameObject;
            }
        }
        return null;
    }
}