Caster

Casters, once called "spell guns," are magical weapons which fire shells that have the effects of spells. When the mana started to run out people started putting the spells in cartridges. Each of the 20 different types of cartridge has a diffrent effect. Caster shells are rare and expensive, and few wizards still know how to enchant them.

 Caster (for Tri-Stat)

  Item of Power 1x3 = 3
    Easy To Lose 
    Special Attack "#3 shell" 1x4+ 1 = 5  (60 damage attack)
      Affects Incorporeal
      Concealable   
      Hand-Held 
      Limited Shots 2x-20 = -40  (3 shots)
      Unreliable 
    Alternate Attack "#12 shell" 1x1 = 1  (20 damage attack)
      Incapacitating
      Limited Shots 2x-20 = -40  (3 shots)
      Unreliable  


Notes:

I was generous with the number of shots. Usually a character will only have one or two shells of a given type. There are obviously many more types of cartridge available, but they are hard to get and characters should be required to undertake a minor quest to acquire the very rare and powerful ones, like a  #4 or #13.

Another way to write this weapon up is to treat the caster as a Major Gadget (or two) and then write up each cartridge as a different Special Attack.