The issues are the following:

1. The Folly (you know, the weapon whose actual name I don't know) can be stacked in stacks greater than 1.
2. The only ammunition accepted by the Folly is Starmetal Silver Bullets.

On a completely unrelated note, what software should I use to create 3d models for Minecraft?

The thing is also not craftable and the one shell that does work doesn't do what it's supposed to. That's because the folly isn't finished.