I do this with my necro, I labeled them as "Caster Shells" as a little tribute, and use them in a revolver. And for the mechanics I just set it up like this: It's an enchanted item, weapon5, single target, with 5 uses per session. So let's just say I have 5 bullets made and in my pocket.
Or you could just say it's 1 bullet that fires 5 times before it runs out, I personally like the first way better.