You can get online and get in touch with companies that sell gun kits. The net result at the end of that is a completely legal, unregistered, untraceable gun.
Ostensibly this is entirely legal. There is a legitimate loophole in the ATF laws that make it completely kosher for you to roll your own. If you have the smarts, you can make a gun. The only catch is that you cannot legally transfer ownership of said gun, ever. It's yours. When you die, it goes in the casket with you. And whereas most of us are law-abiding citizens, and more than a few of us are tool-happy nuts who like to make stuff, that's cool. I recently heard a news story where some curious experimenters fired up their 3-d printer and made some crucial gun components by simply printing them, just to see if they could and make it work. (They could, which startled several interested parties)
It's even okay for companies to sell kits that take some of the especially heavy lifting off the builder: a kit for the lower receiver, the main part of the gun that everything else connects to, starts with a solid piece of metal.
What's wrong with this picture? Not a thing. What you get when you purchase the kit is exactly that: a solid piece of metal, along with detailed, measured drawings showing exactly where and how deep to machine the metal to make it into a functional lower receiver to which other parts can then attach. It takes quite a bit of work, some machinery setup and a fair amount of patience. You have to want it to spend that kind of time.
You can purchase an additional array of supplies which attach to the receiver blank: jigs and guides that make it much easier to machine out the receiver's assorted recesses. Still okay, and still completely within the bounds, I think, of the law.
You can purchase a bulk order of 100 kits at a 30% savings.
Purchasing one, two, even five of these kits is easily passed off as "for personal use only." How can you defend a bulk purchase of 100 kits? What possible use can anyone have for 100 semi-automatic rifles, 1911 automatic pistols, really 100 of anything that isn't shares of stock, money or socks?
See, at this point I think there really is a problem with this picture. I'm actually halfway inclined to buy a kit myself, simply because it is up to me, and not the government, whether I own a gun or not. But damn, a bulk discount for buying 100 kits at a time just shouts, "Arrest me because I am a dangerous nutcase."
Please note that I am not providing links to any of the companies that sell these kits. Because I consider what they're doing to be so iffy from a moral standpoint, I won't make it a no-effort jump to satisfy your idle curiosity. If you go looking for them, that will be on your time and on your own head.