Everyone is missing the most important point.
The ATF is not permitted to legislate (i.e. make rules for citizens to follow) and no amendment was ever approved to give it such power. Thus, it is unconstitutional (and thus illegal) for the ATF to make any rules regarding our right to keep or bear arms (or anything else). It does not matter if you call it a rule, a regulation or a law (these are legislative prerogatives only). The ATF at best is an enforcement agency only (which is what the executive branch is supposed to be). This means the rules we follow either come from Congress (at the federal level) or we pass constitutional amendments. You are familiar with the term "legislating from the bench", well the alphabet soup bureaucratic executive agencies can't legislate from the executive either. They do, but these are de facto rather than de jure acts of government, and they are completely unconstitutional.
Specifically, ATF regulations violate Article I, Section I and the Tenth Amendment of the Constitution (read them side-by-side).
This is not an opinion, it is an absolute fact (so long as we're using the rules of the English Language), and it's not hard to understand if someone (like me) tells you where to look.
I am familiar with Congress passing the unconstitutional/illegal legislation delegating authority to the executive branch and the case(s) where the Supreme Court rubber stamped the legislation: Fordney–McCumber Tariff and J.W. Hampton and Co. v. United States (also unconstitutional).
It would literally require a constitutional amendment in all three cases for the ATF rulemaking process to become legal (i.e. for Congress to pass such a law delegating its power, for SCOTUS to give a binding opinion regarding it and for the ATF to ultimately issue such a regulation). There is no constitutional basis for the ATF prescribing rules after merely posting them for 90 days. Even congress can't do this (because of the Second and Tenth Amendments).
Please do not dismiss this out of hand. No matter how futile it might appear to change our system, it should nonetheless be stressed in every conversation regarding the ATF creating rules we need to follow (or any other situation in which we experience government overreach, otherwise we're just showing the powers-that-shouldn't-be that we're illiterate when it comes to the Constitution.
This is also true of executive orders. There is nothing in the constitution allowing laws, rules, regulations etc. to be passed by the president (and, again, the Tenth Amendment prohibits this). EO's are for the president to issue to his employees in the executive branch and nothing else (in other words, it's purely administrative within the executive branch of government (or where it has constitutional jurisdiction over state and local agencies), but they cannot contradict the constitution which always supersedes everything according to the Constitution [i.e. Article VI and the Tenth Amendment]).
If people brought these points up at every turn we might finally reach a boiling point after seeing how many areas of our lives are affected by this. But the greater point is that 99.9% of Americans fail to understand the Constitution, and if we talked about this more, perhaps we can change that one day. Remember, the Constitution was written for the layperson. It is not written in legalese so there's no excuse to be ignorant of it. But if you wait for our "finest" educational institutions to point this out, you'll be waiting forever.
Again, what I am stating is a fact, and I challenge any lawyers on here (or anyone else) to produce a sensible counterargument (because there isn't one). I'm not trying to "win the internet" as someone once suggested of me. I am merely trying to help my fellow Americans understand how, as Jefferson put it, the Constitution became "a mere thing of wax in the hands of the judiciary which they may twist and shape into any form they please" — Jefferson's letter to Virginia State Supreme Court Judge Spencer Roane, Sept. 6, 1819
