What I think you should do, is to change the flying_missle to either a white string of light, or a real bullet. And the object should be a bullet, only the inventory_item should be a cartridge.
Atleast that's what I think. And to implant an awesome musket shot effect, which I am working on at the moment, not that I'm making any great progress on it. But yea, I'd love to see it in this mod.