"a key part of a semi-automatic weapon called the lower receiver. That part, which comprises most of the body of a gun, is the most regulated element of a firearm. Print a lower receiver, and you can buy the rest of a gun’s components off the shelf without an ID or waiting period."
That's an easy fix I would think. But I'm surprised that the barrel isn't just as highly regulated.
Rifled barrels which have been machined to tight tolerances are non-trivial to make. But a smoothbore barrel is just a tube. For plenty of low-power cartridges, it doesn't even need to be metal.
Purpose. If the police can prove or otherwise convince a judge that the purpose of that metal tube is to serve as a gun barrel, then it's a gun barrel. Circumstances would go a long way in these cases. Say, if the barrel is mounted on a gun. If the owner is known to be making a gun. If the owner has no reason whatsoever to explain why he has a precisely rifled metal tube and a bunch of gun parts on him.
That's an easy fix I would think. But I'm surprised that the barrel isn't just as highly regulated.