The easier thing to do is add more global variables if you have different weapons. pistolAmmo, shotgunAmmo etc. You can have another variable for 'currently equipped weapon' where 0 is pistol and 1 is shotgun. You subtract from ammo depending on the equipped weapon.
When you use instance variables like that all you're doing is moving what could be a global variable into a more difficult way of doing the same thing because now you have to pick instances.
You could think about using instances and variables if your items and ammo are being stored in an inventory array, then they could relate easier but I think you're not using an array.