Okay, there's actually a BIG problem with that as an Item of Power: No Refresh costing powers in the Dresden Files have limited uses. If it's got limited uses, you definitely shouldn't be charging them Refresh for it. Just make them spend a Fate Point to use a bullet when they do so, that's how the Swords of the CDross work. In fact, the Swords of the Cross do literally the exact same thing...and have unlimited uses.
Now, if the Guns do something else, even without those bullets, charging for that is, of course, perfectly reasonable, but charging for the bullets is unnecessary and somewhat unfair.