Automatic shooting

I am making an FPS so I have a shooting mechanism, however it is single-fire (meaning you cannot hold down the lmb (left mouse button) to shoot constantly). I'm a beginner so I suck at C#, so can you guys modify this code for a assault rifle?

using System.Collections;

using System.Collections.Generic;

using UnityEngine;

public class ShootDemo : MonoBehaviour 

{

    public Rigidbody projectile;

    public float speed = 500;

    // Use this for initialization

    void Start () {

    }

    // Update is called once per frame

    void Update () {

        if (Input.GetButtonDown("Fire1"))

        {

            Rigidbody instantiatedProjectile = Instantiate(projectile, transform.position, transform.rotation)

                as Rigidbody;

            instantiatedProjectile.velocity = transform.TransformDirection(new Vector3(0, 0,speed));

        }

    }

}

Any answers will be appreciated greatly.

Thanks!