Which is why my advice is to contact the people who will be arresting you if they don't like what you build.
I think we should be allowed to own and/or make magazines of any capacity but that's just my opinion and when it goes to court no judge will care what I think or what anyone else on this site thinks, they will only care what they interpret the written law to say.
If you don't want to contact the firearms lab then I would contact someone like Rick at ATRS who is a licenced firearms manufacturer and ask what he had to go through to legally manufacture his own pistol magazines. I guarantee it was not as simple as building them and putting a stamp on them, I'm pretty sure he needed to submit a prototype and have it approved. He showed me the first ATRS pistol mag he built and told me it cost him something like $10000 for the first one. I think he asked me somehing like, "wanna see the most expensive magazine in the world?"
Also remember that if you copy another design you may be looking at patent infringement, you can't just 3d print whatever you want unless you design it yourself and when it comes to firearms parts you need to consider that people who legally make firearms parts are licensed to do so not just joe blow with a lathe, milling machine, or 3d printer.