Distributing 3D models, whatever they’re for, can’t be made illegal. Partially because it’s digital media and thus regulated as free speech, and partially because it’s a digital file transmitted from one computer to another. Which is basically impossible to regulate. That said, why is this a concern? It’s not like it’s illegal to own a gun, only illegal to use it improperly. You can make it illegal to distribute the file, but if someone wants to commit a crime with a gun, what’s to say they won’t commit a crime by downloading the gun model too?