How do I assign values to variables in the Update function only ONCE?

Hello there!

I'm making a game that has a weapon customization system. This way a player can alter their weapon's statistics to their liking with different attachments. Every attachment is a game object (prefab) that has their own statistics, that are connected to the weapon's script using interfaces. Here is what I've got so far:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 
 
 
 public class BRN180 : MonoBehaviour
 {
     
      public string Type = "Brownells BRN-180 16 inch";
      public float Weight = 2.0f;
      public float Cal = 5.56f;
      public float baseDamage = 20.0f;
      public float MOE = 1.5f;
      public float reloadTime = 3.2f;
 
 
 
     public GameObject opticUsed; 
     public GameObject opticObject;
      Optic opticStats;
 
     public GameObject magazineUsed; 
     public GameObject magazineObject;
      Magazine magazineStats;
 
      public GameObject stockUsed; 
     public GameObject stockObject; 
      Stock stockStats;
 
 
 
     private SpriteRenderer partRenderer; 
     private SpriteRenderer objectRenderer;
     
     
     void Start()
     {
     }
 void OnGUI(){
      GUI.Label (new Rect (200, 100, 300, 60), "Weapon's name: "+Type);
   GUI.Label (new Rect (200, 150, 300, 60), "Weapon's weight: "+(Weight+opticStats.Weight+magazineStats.Weight+stockStats.Weight)+" kilogramms");
   GUI.Label (new Rect (200, 200, 300, 60), "Weapon's damage: "+(baseDamage*Cal));
   GUI.Label (new Rect (200, 250, 300, 60), "Weapon's accuracy: "+(MOE-opticStats.Accuracy*stockStats.Stability)+" MOE");
   GUI.Label (new Rect (200, 300, 300, 60), "Weapon's capacity: "+(magazineStats.Capacity)+" rounds");
 
 
 }
     // Update is called once per frame
     void Update()
     {
             opticStats = opticUsed.GetComponent<Optic>();             
             partRenderer = opticUsed.GetComponent<SpriteRenderer>(); 
             objectRenderer = opticObject.GetComponent<SpriteRenderer>(); 
             objectRenderer.sprite = partRenderer.sprite; 
 
 
 
             magazineStats = magazineUsed.GetComponent<Magazine>(); 
             partRenderer = magazineUsed.GetComponent<SpriteRenderer>(); 
             objectRenderer = magazineObject.GetComponent<SpriteRenderer>(); 
             objectRenderer.sprite = partRenderer.sprite; 
 
 
 
             stockStats = stockUsed.GetComponent<Stock>(); 
             partRenderer = stockUsed.GetComponent<SpriteRenderer>(); 
             objectRenderer = stockObject.GetComponent<SpriteRenderer>(); 
             objectRenderer.sprite = partRenderer.sprite; 
 
     
     }
 }

This displays the current statistics of the weapon, but the statistics don't actually change... If I try calculating them as I do when I write them on a Label, it keeps getting recalculated each frame.

So, how do I go about changing stats, only when an attachment changes?

Thank you in advance!