I believe you can do this by buying a 80% lower receiver and milling it out yourself. Then, not serializing it and selling it. I think you’re supposed to legally serialize it especially if you plan to sell it. Then if you start making an unreasonable amount of these, you would need at minimum an FFL license.  It’s all so convoluted and stupid. I’m not sure how we got to the point where we write legislation that is in direct contradiction with our Constitution, without batting an eye. Makes me very sad. SHALL NOT BE INFRINGED.