First of all, there is Law. The Law is passed by the legislature, and it directs some government agency to do something. Which leads to…
Regulations. Regulations are what the tasked agency writes to tell the public exactly how they will do something. Which leads to Policy/Procedure which is how the worker bee in that agency performs their tasks.
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But, the thing that really counts is Case Law. (IANAL warning in full effect here.) Because laws/regulations/policies are written by humans, there will always be some ambiguous parts. Which generally leads to lawsuits.
When the Judge rules in the case, it can forever alter the way the law is implemented. A judge might write an opinion that includes a legal definition of a term of art that was not part of the Law or Regulation. Or, they may declare the law does not mean _______ as the Agency interpreted it, but instead means ________. Could be anything.
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The lesson here is reading the law is one thing, reading the regulations is another, but the thing that really makes a difference is the rulings on suits brought against the law. That tends to be the final word.
— CBMTTek

US law is based on the Constitution, everything flows from it, or should. Any law, regulation, or procedure should be consistent with the Constitution.

In the best of worlds before a bill was even presented to the legislature it would be vetted by people that actually care about the constitution(s) of the US and the State. Unfortunately this is not the case. Or if it is done it is such a poort analysis as to make no nevermind.

“We read the Constitution and our bill is Constitutional” does not mean that it is Constitutional. Just that the legislator wants it to be.

The next stop for checking if a bill is Constitutional is when the bill moves to the next chamber of the legislature. Again we have the issue of the situation where the people that want the bill to pass will just claim it is Constitutional.

Finally, the President or the Governor should check it for Constitutionality. This very seldom happens.

So now Bill has become a law. It is up to the executive branch to enact that law. AS CBMTTek says, that requires interpretation. If a word or phrase is not defined by the Law, it must be defined by regulation.

Even if the word is well defined, “of the People”, the regulators might decide to twist the words. So we end up with “of the Militia/State” where the clear meaning is “People”.

The regulators are not responsible for deciding if a law or regulation is constitutional, they are only responsible for making sure that the regulation is supported by law. Or that they can twist the words to match what is supported by law.

Thus we end up with everybody knowing what a frame or receiver is. But the ATF gets to say that something that is not a receiver becomes one if you drill a hole in it, or make a dimple in the right place. An object becomes a firearm, underlaw, when it is a frame or receiver, not when it is one hole away.

Consider an AR-15 receiver v. an M16/4 receiver. That AR-15 receiver is not a machine gun. It only becomes a machine gun after that hole is drilled. Only when it is capable of receiving all of the fire control group that complete it as a machine gun.

If that AR-15 doesn’t have the space to hold an auto-sear, even if the fourth hole is drilled, it still isn’t a machine gun, even if the ATF says it is.

Once the regulations are written, somebody has to write the procedures used to follow the regulations. The law says that the FFL has to maintain a record of all firearms sold. The GCA of 1968 doesn’t say anything about having to keep a record of all firearms received. It is the regulations that describe what information has to be kept, how it has to be kept, and how the ATF (regulatory agency) gets to verify that information.

There is no Constitutional authority for the ATF to inspect an FFLs books. That would require probable cause. What has happened is that the state has made it a condition on holding an FFL to give up your fourth amendment rights. Sort of like there is no federal authority to set speed limits. It is just that the federal government will withhold funding if the state doesn’t set the speed limits the federal government wants.

When a government agent goes to do something, they are suppose to follow the procedures laid out for them by their superior. This tells them how to discover issues that can lead to enforcement actions. When the ATF agent shows up at the FFL and asks to see their records, they are suppose to follow a procedure.

Once they start the examination, they are suppose to be following the procedures and guidelines that were given to them. When we see multiple ATF agents using digital cameras to capture information from 4473s, that strongly suggests that it is a procedure they were told to follow.

As an example of procedures being followed, consider a YouTube video of CPS “visiting” a household. CPS had been told to leave the property when they first showed up. So they got the local police involved. The police arrived and found a gate across the drive and the property was properly posted “No Trespassing”.

CPS got out of their car and the cops got out of theirs. They started walking up the drive. When they got to the gate CPS went over and the cops stopped.

The procedures that the cops were following said that they needed probable cause to enter the property. Having some government agent tell them to intimidate the household wasn’t. CPS on the other hand had a procedure which was to force their way into the house via verbal and then intimidation and threats.

These laws, regulations, and procedures lead to conflict between individuals and the government. Individuals in a legal sense so it includes corporations and other such legal entities.

When there is conflict the issue goes before a court. If there are facts to be determined (Did Bob put his hands on Jill?) then either a jury or a judge will make those determination. So a jury might be asked to decide if the accused willfully did something or was it an accident (was she pushed or did she fall?)

Part of the trial/argument stage is to decide what facts are allowed into the case.

Another part of the legal process is when the actual law is challenged. The city says that you are not permitted have a gathering in the city park of more than 25 people without city permission. You stood up on a rock in the city park and started giving a speech. Suddenly there were a few hundred people there listening. The cops arrest you for having a gathering of more than 25 people without permission.

In court the facts are clear. There was a gathering of more than 25 people. It is to be determined if you are legally responsible for causing that gathering. If you are, then you will be found guilty and a punishment applied. On the other hand, you could challenge the law as a violation of your right to free speech.

It is also possible for a person to challenge a law directly. To file a lawsuit alleging that some law, regulation, or procedure is in violation of a higher law. This is where we see most of the second amendment cases coming.

Now the Constitution sets up the Supreme court and such inferior courts as are needed. (I didn’t lookup the exact wording). The reason is that under common law, decisions made by the higher courts set precedent for the inferior courts. This is “case law”

If the Second Circuit court says that the second amendment only applies to the right of the state militia to keep and bear arms, all inferior courts to the second circuit must follow that opinion. If the Supreme court says that the second amendment applies to all the people, regardless of their membership in the organized militia, then all the circuit courts and all the district courts and all the state supreme courts and all the courts inferior to the state supreme court must follow that opinion.

This is how we create case law.

When a district court makes a decision it is not case law. It can be used by other district courts but it is not precedent. In addition, decisions/opinions for district courts in different circuits count for even less. It is only courts that are superior to a court that create precedent for the inferior court.

As the courts make their decisions/opinions they are creating case law for themselves and inferior courts. Those opinions should describe what the question is, it should define all the terms within that question, and it should then support those definitions. Finally the court says why the made the the decision they made.

Unfortunately, there are people that conflate the different parts of court decisions. When the court says “this is an example” it doesn’t mean that it is the only or that it is the common, or that it is everything except that single example. But there are people that will take it that way.

We hear that when some moron argues that the second amendment only applies to muskets or only to the militia or any of the other stupid arguments.

The supreme court works hard at defining terms. Legislators, not so much.

A lawyer once looked over a contract I had written. He told me that it was good but that it was too clear. As he put it, lawyers like terms in contracts that can lead to conflict, because that’s when they make money. While to us a good contract is a way of making sure there is not conflict later.

It is this case law that creates the legal definitions of terms. It is this case law that tells us how to apply the different terms. It is case law that tells us what the actual law says or does.

ObamaCare doesn’t penalize you for not having health insurance, it just increases your taxes. This is case law, even though the legislature said it was a penality.

The legislature said that a firearm is anything that expels a projectile by means of an explosive. It also includes those things that can be “readily” converted to expel a projectile by means of an explosive.

No court has defined what “readily” means. Since there is no case law the regulatory agency can set the definitions. As soon as they set those definitions then it can be challenged and we can create case law.

The ATF is also trying to change the definition of a frame or receiver. They are perfectly within the scope of the law to say that something is or is not a receiver. They can even say that something can be converted into a frame or receiver. What they can’t do is change the law to claim that something that is not a frame or receiver should be treated as if it is a frame or receiver.

If something can be readily converted into a frame or receiver then it is not a frame or receiver. If it is not a frame or receiver then it is not a firearm per the GCA of 1968.