Best way to interchange variables of a single script?

I think you'd really like Scriptable Object's. Brackeys has an amazing video on this that really puts it all in plain English too.

If those don't float your boat, I would also suggest trying out a Strategy Design Pattern . The purpose of this is to provide an easy way to make many things that do the same thing, differently. In plain English: QuickSort and MergeSort both sort a list, but they go about doing it differently. In your case, SMG and Pistol both fire bullets, but may deal damage differently.

To get you started with a basic Strategy Pattern:
IWeapon.cs

 //you'll want an interface as the main type to allow you to swap out the damage of guns.
 public Interface IWeapon {
     //gets and sets the damage the weapon deals.
     public float getDamage();
     public void setDamage(float damage);
     //gets and sets the rate of fire
     public float getRateOfFire();
     public void setRateOfFire(float rate);
     //gets and sets for whether the weapon is automatic.
     public bool getIsAutomatic();
     public void setIsAutomatic(bool isAuto);
 }

Now, lets create a base weapon type that contains the relevant info for a pistol:
Pistol.cs

 public class Pistol : IWeapon {
 // because this implements the IWeapon interface, you also need to include all its methods.
     
     float damage = 10;
     float rateOfFire = 15;
     bool isAuto = false;
 
      public float getDamage() {
         return this.damage;
     }
      public void setDamage(float damage){
         this.damage = damage;
     }
      //gets and sets the rate of fire
      public float getRateOfFire(){
         return this.rateOfFire;
     }
      public void setRateOfFire(float rate){
         this.rateOfFire = rate;
     }
      //gets and sets for whether the weapon is automatic.
      public bool getIsAutomatic(){
         return this.isAuto;
     }
      public void setIsAutomatic(bool isAuto) {
         this.isAuto = isAuto;
     }
 }

When you want to create say, SMG or Rifle now, all you need to do is implement the IWeapon interface.

To use these, a quick code snippet would be like so:

     public IWeapon weapon = new Pistol();
     public Animator playerPoses;
     public gameObject weaponModel;
     
     public someShootingScript _shootingScript; //the actual script that shoots the gun and has the damage per bullet.
     
     void Start() {
         _shootingScript.setModelAndAnimator(weaponModel, playerPoses);
         _shootingScript.setDamage(weapon.getDamage());
         //... and so on
     }
 
     void changeToWeapon(Animator pose, GameObject model, IWeapon weapon) {
         //pretty much just repeat Start()
     }

I hope this helps!