Am I doing this correctly?

So will this script;

Detect when the bullet enters the enemy’s collider

Subtract the bullet damage from the total enemy health

Destroy the enemy when the enemy health is less than or equal to 0:

using UnityEngine; 
using System.Collections; 

public class M9Damage : Monobehaviour {
    
    public GameObject M9_Round; 
    public float Enemy_Health = 100.0f; 
    public float M9_Damage = 24.0f; 
    
    
    void EnemyDeath () 
    {
        Destroy.gameObject; 
    }
    
    
    void OnCollisionEnter (Collision col) 
    {
        if (co.gameobject.name == "M9 Round")
        {
            Enemy_Health -= M9_Round; 
            
        }
    }
    
    void Update () 
    {
        if (Enemy_Health <= 0) {
            EnemyDeath (); 
        }
    }
}

The main issue I see is that:

Destroy.gameObject;

Is not valid syntax.

It should be:

Destroy(gameObject);

Secondly, you don’t need to check health every frame, because health is not changing every frame. You can call a function like this to change the health value:

private void TakeDamage(float damage)
{
    Enemy_Health -= damage;
    if(Enemy_Health <= 0)
    {
        Enemy_Health = 0;
        EnemyDeath();
    }
}

using UnityEngine;
using System.Collections;

public class M9Damage : Monobehaviour {
	// not required
	// public GameObject M9_Round; 
	public float Enemy_Health = 100.0f; 
	public float M9_Damage = 24.0f; 


	void EnemyDeath () 
	{
		Destroy.gameObject; 
	}


	void OnCollisionEnter (Collision col) 
	{
		// typo
		// if (co.gameobject.name == "M9 Round")
		if (col.gameObject.name == "M9 Round")
		{
			// wrong item
			// Enemy_Health -= M9_Round;
			Enemy_Health -= M9_Damage;
		}
	}

	void Update () 
	{
		if (Enemy_Health <= 0) {
			EnemyDeath (); 
		}
	}
}

using UnityEngine;
using System.Collections;

 public class M9Damage : Monobehaviour {
     
     public GameObject M9_Round; 
     public float Enemy_Health = 100.0f; 
     public float M9_Damage = 24.0f; 
     
     
     void EnemyDeath () 
     {
         Destroy(gameObject); 
     }
     
     
     void OnCollisionEnter (Collision col) 
     {
         if (col.gameObject.name == "M9 Round")
         {
             Enemy_Health -= M9_Damage; 
             
         }
     }
     
     void Update () 
     {
         if (Enemy_Health <= 0) {
             EnemyDeath (); 
         }
     }
 }

Very simple code, but the way you are doing this will simply cause a lot of trouble once the game gets bigger, You have a damage script for a round, what happens if you get different types of rounds, or even different types of objects ? You will create a script for each type of object? That’s a no-no in development world. You need to learn about polymorphism. I would suggest you to learn at least basics in C# before you jump in into creating more advanced games like FPS.