The lower part of the AR is the only part that needs to be registered with the ATF. It is also the only component that you can’t buy online. However, homemade weapons are allowed to bypass this law. This means you can now get an AR15 for half the price, and without the registration hassle, assuming you have a 3D printer at your disposal.
In the video below, you’ll see a printed lower tear through 600 rounds without failure.