Shooting

//in player behaviour

public GameObject bulletPrefab;
 public float bulletSpeed = 100;
 private bool isShooting;

 void Update() {

     isShooting |= Input.GetKeyDown(KeyCode.Space);
 }

 private void FixedUpdate() {

     if (isShooting)
     {
         GameObject bullet = Instantiate(bulletPrefab, transform.position + transform.forward, transform.rotation);
         Rigidbody bulletRB = bullet.GetComponent<Rigidbody>();
         bulletRB.velocity = transform.forward * bulletSpeed;
     }
     isShooting = false;
 }

//in bullet behaviour and attached to bullet prefab
public class BulletBehaviour : MonoBehaviour
{
    public float onScreenTime = 3;
    void Start() {

        Destroy(gameObject, onScreenTime);
    }

}