An ambiguity or inadequacy in a system, such as a law or security, which can be used to circumvent or otherwise avoid the purpose, implied or explicitly stated, of the system.
Since the system, in this case the ATF rules, were designed to allow individuals to make their own guns, this isn't a loophole. Like the "gun show loophole" this rule was intended to allow the kind of activities considered objectionable by the group bringing the lawsuit.

Neither 80% lowers not being considered firearms nor the ability of an owner to privately sell a gun, their personal property, were planned to be regulated by federal government. If one is a prohibited person such as a felon, domestic abuser or has been dishonorably discharged from the military, it is still illegal for them to manufacture or possess a firearm.

IMO, laws best serve as criteria for juries and courts to use to determine the guilt of the accused. Laws that are aimed at enforcing compliance by the citizenry to standards of behavior held by the legislature (or actually only by a majority thereof) and enforced by an, at times, hostile police officer or unit are less than ideal.

It is undesirable to have government in the role of controlling the behavior of private citizens. Laws intended to provide such controls are often impossible to enforce and frequently lead to bigger problems.

I've read stories about homemade (and illegal) gun builders working on every continent except Antarctica and in dozens of countries.

I know the reasoning behind making it illegal for certain people (convicted criminals, etc.) to possess or manufacture guns. I also know that someone determined to engage in crime will find a way to do what they want to do and get what they need to do it but it goes without saying that no one wants to arm violent criminals. I'm onboard with that. However, I do think some of our, and by "our" I mean humans, thinking needs maturing. Our thinking on crime needs to take an evolutionary step.

Guns have many valid and legal good uses such as competitive shooting, hunting and collecting in addition to self-defense. IMO requiring someone to jump through some hoops to acquire a gun is the wrong direction to take. Why should an individual need to prove that they should be allowed to do such basic things? We have the background check system for dealer sales of new and used guns for this reason. The existing system looks at state and federal data for a reason to deny the transfer. The burden on the government to produce the reason for the denial and the individual can challenge the decision.

Today, we are hearing more about ghost guns. The fact is whether it suits us or not neither magic spells, Jedi skills nor a blessing from Elrond are required to manufacture a gun.

STrongly pro RKBA myself.

We made laws making murder, assault... illegal. Criminals continued to murder and they used guns to murder often.
We made it illegal to get a gun if you're a certain level of convicted criminal. Convicted criminals continue to get guns and continue using them to assault and murder. One way is making their own guns. Another is stealing guns from others, (that's also illegal.)
Now we are looking to make getting gun parts illegal.
What I'm seeing is that we are repeating the same actions with the same goals without achieving the desired result.

Crime leads to investigations which lead to arrests. Arrests generally lead to convictions and prison. Then 5 to 15 years later these criminals are released into the same environment where getting guns is illegal but still common for criminals to get them. IMO that's not reasonable.

Maybe we could try a few things that embody a bit more hope of success. Maybe keep these folks in prison until they're no longer a threat. And maybe if we could find what leads these folks to such crimes, we could provide support, guidance and direction and prevent them from getting into crime.

At this point I'm beginning to doubt that our governments, federal, state and local, really want to solve this problem. Which tact is worse? One group wants to blame guns and make them illegaler. Another group wants to blame drugs and make them illegaler. I've written about the results of making guns harder to get and how that's not working. Blaming drugs isn't working. Once we figured out that lots of drugs are smuggled in, we began taking the "WAR" against drugs to other countries. We send arms, advisors and troops in boats, trucks and helicopters who are armed, BTW. there is some evidence that some of those arms are acquired by gangs running the drugs into the US.

But, in spite of all that, I don't see much evidence that at least the federal government really wants to stop gun violence. Gunrunning is a business and whether it's legal or illegal (or in some not well defined gray areas) people are making money from running guns. Running guns around the corner or around the globe is netting some folks a profit.

Some of the truest words I've ever read on gun running are from the movie, Lord of War:
"While private gunrunners continue to thrive, the world's biggest arms suppliers are the US, UK, Russia, France and China. They are also the five permanent members of the UN Security Council."

this could happen

https://www.homedepot.com/p/Southland-3-4-in-x-48-in-Black-Steel-Pipe-584-480HC/100192945
Gun - a shotgun shell is slightly under 3/4"

https://www.homedepot.com/p/Everbilt-10-x-2-in-Phillips-Pan-Head-Zinc-Plated-Sheet-Metal-Screw-50-Pack-801682/204275095

Gun - its point is sufficient to fire a primer.

https://www.homedepot.com/p/Swaner-Hardwood-2-in-x-6-in-x-8-ft-Walnut-S4S-Board-OL08051696WA/311941170
Gun - many shotguns are made with walnut.

https://www.homedepot.com/p/Everbilt-Spring-Assortment-Kit-84-Pack-13554/203133714
Gun - all firearms have springs.

In fact as all of these are more than you would need for 1 gun, the charge would be building guns with the intent to distribute.