You can create muzzle flashes easily by using textured planes in front of the camera. Alternatively you can use animated meshes, or particle effects. No matter which effect you will choose, you can show the effect by using a simple script that is sitting on the muzzle gameobject like:

public class MuzzleFlash : MonoBehaviour
{
    private float _muzzleFlashEnd = 0;

    public void Show(float time)
    {
        //enable renderer/animations
        gameObject.SetActiveRecursively(true);

        //add some variations
        transform.localRotation = Quaternion.Euler(0, 0, Random.Range(0, 360));

       //set a duration for the effect
        _muzzleFlashEnd = Time.time + duration;
    }

    private void Update()
    {
        if (_muzzleFlashEnd < Time.time)
        {
            gameObject.SetActiveRecursively(false);
        }
    }
}

Now activate the effect by something like

if(input.GetMouseButtonDown(0))
{
   _myMuzzleFlash.Show(0.1f);
}

The gameobject will activate and deactivate itself when the effect is over. That way it will not consume any resources when not in use.