Am I doing this correctly?

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.

https://www.tutorialspoint.com/csharp/csharp_polymorphism.htm https://en.wikipedia.org/wiki/Composition_over_inheritance