Solution: write another law.
Result: politicians pat themselves on the back for doing something, non-felons can no longer build DIY firearms, felons continue to ignore the law.
Sigh.
With the amount of uppers that are in the wild, and the fact that a lower can be made from either an 80% part, or 3D printed from scratch by pretty much anyone with two working hands means trying to stop this is like pissing in to a hurricane.
Google FGC-9. Even the barrel can be made at home, rifling and everything, ivanthetroll has videos/files online documenting the process. If Henry Colt could produce a firearm using 100 year old tools and technologies, it's comical to think you can stop people doing the same using 2021 technology.
Perhaps it's time to consider bringing up the poorest members of society out of the abject poverty, that will do a whole lot more to reduce gun violence than passing even more ineffective gun control laws.