Unity - Destroying game object with tag "enemy" with projectiles, ned help.

Hey, so I feel that it's not a good sign I need a lot of help with this project XD Basically, I am making a 3rd person shooter, where the player is a robot who fights other robots by shooting them. However, the shots I fire do not destroy the enemies (labelled by tag "Enemy") but also, when they run into my robot, they then get destroyed. I was wondering how I can get it so the enemies get destroyed by my shot but don't get destroyed when they run into me. Here is my script:
using UnityEngine;
using UnityEngine.UI;

public class TankShootingV2 : MonoBehaviour
{
public int m_PlayerNumber = 1; // Used to identify the different players.
public Rigidbody m_Shell; // Prefab of the shell.
public Transform m_FireTransform; // A child of the robot where the shells are spawned.
public AudioSource m_ShootingAudio; // Reference to the audio source used to play the shooting audio. NB: different to the movement audio source.
public AudioClip m_ChargingClip; // Audio that plays when each shot is charging up.
public AudioClip m_FireClip; // Audio that plays when each shot is fired.
public float m_MinLaunchForce = 15f; // The force given to the shell if the fire button is not held.
public float m_MaxLaunchForce = 30f; // The force given to the shell if the fire button is held for the max charge time.
public float m_MaxChargeTime = 0.75f; // How long the shell can charge for before it is fired at max force.

private string m_FireButton; // The input axis that is used for launching shells.
private float m_CurrentLaunchForce; // The force that will be given to the shell when the fire button is released.
private float m_ChargeSpeed; // How fast the launch force increases, based on the max charge time.
private bool m_Fired; // Whether or not the shell has been launched with this button press.





private void Start()
{
// The fire axis is based on the player number.
m_FireButton = "Fire" + m_PlayerNumber;

// The rate that the launch force charges up is the range of possible forces by the max charge time.
m_ChargeSpeed = (m_MaxLaunchForce - m_MinLaunchForce) / m_MaxChargeTime;
}


private void Update()
{
// The slider should have a default value of the minimum launch force.


// If the max force has been exceeded and the shell hasn't yet been launched...
if (m_CurrentLaunchForce >= m_MaxLaunchForce && !m_Fired)
{
// ... use the max force and launch the shell.
m_CurrentLaunchForce = m_MaxLaunchForce;
Fire();
}
// Otherwise, if the fire button has just started being pressed...
else if (Input.GetButtonDown(m_FireButton))
{
// ... reset the fired flag and reset the launch force.
m_Fired = false;
m_CurrentLaunchForce = m_MinLaunchForce;

// Change the clip to the charging clip and start it playing.
m_ShootingAudio.clip = m_ChargingClip;
m_ShootingAudio.Play();
}
// Otherwise, if the fire button is being held and the shell hasn't been launched yet...
else if (Input.GetButton(m_FireButton) && !m_Fired)
{
// Increment the launch force and update the slider.
m_CurrentLaunchForce += m_ChargeSpeed * Time.deltaTime;
}
// Otherwise, if the fire button is released and the shell hasn't been launched yet...
else if (Input.GetButtonUp(m_FireButton) && !m_Fired)
{
// ... launch the shell.
Fire();
}
}


private void Fire()
{
// Set the fired flag so only Fire is only called once.
m_Fired = true;

// Create an instance of the shell and store a reference to it's rigidbody.
Rigidbody shellInstance =
Instantiate(m_Shell, m_FireTransform.position, m_FireTransform.rotation) as Rigidbody;

// Set the shell's velocity to the launch force in the fire position's forward direction.
shellInstance.velocity = m_CurrentLaunchForce * m_FireTransform.forward; ;

// Change the clip to the firing clip and play it.
m_ShootingAudio.clip = m_FireClip;
m_ShootingAudio.Play();

// Reset the launch force. This is a precaution in case of missing button events.
m_CurrentLaunchForce = m_MinLaunchForce;
}

private void OnCollisionEnter(Collision collision)
{
// You probably want a check here to make sure you're hitting a zombie
// Note that this is not the best method for doing so.
if (collision.gameObject.tag == "Enemy")
{
Destroy(collision.gameObject);
}
}
}

I apologise for asking such a stupid question, I am just really stuck on this.