The tough part about regulating 80% lowers or solvent traps (80% suppressors) is deciding what the limit is for something to be a gun. Some makers of 80% lowers also offer 0% lowers which are just solid blocks of aluminum. If you have more advanced tooling, you can just as easily turn it into an AR lower. Pistols and pistol caliber carbine lowers are already easy to 3D print and reasonably reliable. There's at least two ways I know of to make a rifled barrel at home without needing much knowledge and less than $100 in equipment. There's just no real way to limit this shit because if an 80% lower is illegal, then a block of aluminum should also be illegal. Bar stock and steel tubing would have to be made illegal. Otherwise, makers will just drill 1 less hole and call it a 79% receiver.
The pistol brace stuff has always been kind of borderline, but the ATF didn't give a fuck about them until they were popular. When AR pistols were new, they were generally unliked by the gun community because they weren't practical until some of the more modern pistol braces were invented. Either way, you can still buy all these parts totally unregulated from China because most airsoft guns these days use milspec parts like stocks, grips, and handguards. Since these aren't pressure bearing parts, they can also be 3D printed very easily.
Australia did a mandatory gun turn in and ended up getting less than 30% of weapons in circulation. How do you think Americans will do?