How do I make my weapon use more ammo during its altfire?

https://zdoom.org/wiki/Actor_properties#Weapon.AmmoUse2

DECORATE supports alternate firing modes. A rough example below:

ACTOR MyGun : Shotgun 
   Decal BulletChip
   Inventory.PickupMessage "The overpowered, invisible, and silent Shotgun!"
   Weapon.SlotNumber 3
   Weapon.SelectionOrder 2000
   Weapon.AmmoType "Shell"
   Weapon.AmmoType2 "Shell"
   Weapon.AmmoGive 20
   Weapon.AmmoGive2 20
   Weapon.AmmoUse 1
   Weapon.AmmoUse2 4 //deduct 4 shells from ammo counter, if altfire is used
   States
   { 
   Select:
       TNT1 A 1 A_Raise
   Loop
   Deselect:
       TNT1 A 1 A_Lower
   Loop
   Ready:
       TNT1 A 1 A_WeaponReady
   Loop
   Fire:
       TNT1 A 1 A_FireBullets(2, 2, 7, 10)
       TNT1 A 1 A_ReFire
   Goto Ready
//this is the secondary fire, usually activated by right-mouse click.
   AltFire:
       TNT1 A 1 A_FireBullets(4, 4, 14, 20)
       TNT1 A 1 A_ReFire
   Goto Ready
   }
}

Weapon.AmmoUse2 is a weapon property that defines how much ammo should be deducted. In this, case, 4 shells will be removed.

The AltFire state is the alternate firing state. Here the damage produced is even greater and more "shells" are shot (14 to be exact).

Thanks! It's quite an ambitious first weapon mod, but I got it done thanks to your help!