The federal bureaucrat responds: The “gun” is the thing with the serial numbers on it. In the case of AR-pattern rifles, that’s what’s known as the lower receiver (or just “lower”). It’s the lower part that encases the rifle’s action, where the pistol grip attaches and a magazine goes in.

Everything else is an accessory, even though it actually does the work of a gun, namely loading a cartridge into the action, striking the primer, which ignites the gunpowder, launching the projectile and hot gasses out of the barrel, and, finally, getting ready to do so again.

This is the official “gun” part of an AR. Everything else is really, truly, not considered a “gun.”

The decision to anoint the lower receiver a gun is a momentous bureaucratic decision. It means that everything in the “non-gun” category, which is most of the actual gun, can be bought and sold unrestricted online, like so many shoes on Zappos. Right now, you can buy everything in the “non-gun” category with zero checks online.

Particularly in light of the initial discussion around advanced manufacturing, you might ask: If the “gun” is this relatively small piece of metal or polymer, then why can’t I make it myself somehow?

Let’s answer that via a thought experiment:

Say I took a rectangular hunk of aluminum and machined out a hole like the magazine well of the AR.

Then I machined out the trigger opening.

Then I machined out the base where the pistol grip attaches and every other detail in the “gun” part (see graphic).

At what point did the hunk of metal become a “gun”?

This isn’t mere philosophizing. It’s a real issue in gun regulation. The federal bureaucrat responds again: That piece of aluminum became the “gun” when you got 81 percent of the way to finishing it. After that point, it must be bought and sold as a gun, with all the restrictions that apply.

What does the enormous US market in gun parts do? It sells you an 80 percent finished piece of metal, which is officially not a “gun” (though it really looks like one) on the internet, no questions asked (link provided for reference, not an endorsement). This is known in the gun world as an “80 percent lower.”

The aspiring unregistered gun owner then finishes the last 20 percent at home, with a drill press and/or a milling machine. Combined with the “non-gun” parts bought freely online, they now have a fully functioning AR15. Nobody knows that gun exists, and it is completely untraceable.

Some retailers (such as the one linked above) offer you the entire package: every non-gun piece required and an 80 percent finished “gun” (again, really just the lower receiver), plus they’ll even include some machining jigs to make the finishing easier. This is the IKEA model of firearms, which even has its own IKEA Effect: New gun owners feel disproportionately attached to a gun they “made,” even though really they just put it together, often with the same hex keys you’d use on an IKEA bookshelf. Anyone with a credit card and address can buy this, and nobody on the planet is the wiser, save the retailer, who is not required to report anything, since they’re just selling “accessories.”

The entire gun debate is really about that last 20 percent of the bureaucratic “gun,” and how and where it happens. This is where Defense Distributed really comes in. They sell you a gadget that makes that last 20 percent step easier than doing it yourself, via a computer-controlled milling technology that’s been around forever. It isn’t a fundamentally new gun technology, but it is an important last-mile advance.