How to adjust damage taken to a zombie based on gun displayed on screen?

I am currently working on a FPS game for a school assignment. The main question that i have is what is the most effective way of changing health loss/damage of gun based on weapon displayed using prefab shooting ?
i have already tried using a switch statement and also checking whether the object is active in the hierachy (ie. weapon01.activeInHierarchy).

My Code is Below

using UnityEngine;
using System.Collections;

public class bullet : MonoBehaviour {

public EnemyHealth eh;

// Use this for initialization
void Start () {
	Destroy (this.gameObject, 1);

void Update(){


//function works for zombie bullet collision
void OnTriggerEnter (Collider other) {
		if (other.tag == "Enemy") {
			Debug.Log ("Enemy Hit");
			eh = (EnemyHealth)other.GetComponent (typeof(EnemyHealth));
			eh.ChangeHealth (-20);

		} else {
			Debug.Log ("Didn't hit the Enemy");


create a public variable public float damage

make sure you set the damage in the inspector and update the prefab.

eh.ChangeHealth (damage);

Create multiple bullet types that are instantiated from different guns with different damage values.