I don't know much about gun technology, so maybe somebody can explain why the lower receiver is the controlled part. Why not the barrel? It seems to me that the barrel is the heart and soul of a gun. You can make a gun with just a barrel and nothing else: it's called a cannon.
Furthermore, I would think the barrel would be the hardest part to manufacture, given the forces and temperatures it must endure, and having to be perfectly straight, and rifled. I would imagine that 3-D printed barrels are probably a long, long way off.
3-D printed receivers are already here, and only going to get better and cheaper. If they don't change the regulatory framework to start controlling the rest of the parts, then soon anyone that wants to will be able to make a gun at home and all the gun control laws in the world will just be so much pissing in the wind.
The lower reciever is the part that is seralized and tracked. -Speed
The sooner you fall behind, the more time you have to catch up.