>It would take a lot more legal wrangling to ban things like barrels and small internal parts than it would to just ban the "gun itself", and as far as the law is concerned the lower is the "gun".
Realistically speaking, could this even be accomplished? Anything could be a precursor to small parts, some dude can just file away at metal or 3D print in his garage and nobody would be the wiser. The only hard thing to get is a barrel, but that's only because those machines used to make them aren't exactly easy to by and are super conspicuous.