I might be reading it wrong, but it looks to me like he printed a receiver and placed all the moving parts and the barrel etc. in it. See "I printed a modified version of the lower from cncguns.com"
The receiver is the only part of the rifle that is controlled. Everything other than the receiver can be bought on the internet without background checks or registering of serial numbers.
Yes, I know that part, I was simply pointing out that he did not necessarily print out a barrel or trigger pin or any of the other many parts needed to make a whole pistol.
