Lock rotation of tank barrel

I have created a working prototype for a tank game. The final thing that I cannot get right is stoping the angle of the barrel at a set angle to stop it going stright up looking unrealistic or digging into the ground. My current script is

 using UnityEngine;
 using System.Collections;
 
 public class TurrentMouseVert : MonoBehaviour {
 
     public float horizontalSpeed = 2.0F;
     public float verticalSpeed = 2.0F;
 
 
         
     
     
     
     void Update() 
         
     {
         float h = horizontalSpeed * Input.GetAxis("Mouse X");
         float v = verticalSpeed * Input.GetAxis ("Mouse Y");
         transform.Rotate (v, 0, 0);
 
     
         
         //Debug.Log(transform.eulerAngles.x);
         Debug.Log(transform.eulerAngles.y);
         //Debug.Log(transform.eulerAngles.z);
     }
 
 
 }

I have removed my test part that included mathf.clamp to avoid confusion, please help