C# Camera based Recoil

Hello, I've recently been working on a game in Unity, an FPS (Very original, I know), and am attempting to add in a system by which the camera will rotate up when firing, and smoothly move back to the previous rotation when not shooting.

This is what I'm working with at the moment, any help would be greatly appreciated.

 using UnityEngine;
 using System.Collections;
 
 public class MoveRecoil : MonoBehaviour {
 
     public GameObject PlayerCam;
     public Quaternion startpos;
 
 
     void Start ()
     {
         startpos = PlayerCam.transform.rotation;
     }
 
     // Update is called once per frame
     void Update ()
     {
             
         if (Input.GetKey (KeyCode.Mouse0)) {
             PlayerCam.transform.Rotate (Vector3.right, -.5f);                                                                                                                    
         } 
         else {
             PlayerCam.transform.rotation = Quaternion.Lerp (PlayerCam.transform.rotation, startpos, 0);   
         }
     }
 }