I agree the argument upthread is weak, but your defense of the ACLU is kinda weak, too.

The ACLU's official legal stance on the 2nd Amendment is that it protects a collective right, not an individual right. And that collective right is, as a practical matter, reposed with the state. See https://www.aclu.org/other/second-amendment

In other words, the ACLU sees the 2nd Amendment as a states' rights issue. But we we all know that states' rights is a loaded term in American political discourse. It was originally coined to defend Federal laws which affirmed property interests in slaves traveling in Northern states. In other words, the issue of the day was the slave holding states' rights to pass laws defending the property rights of its slave holders. And the argument was that if those property laws could be ignored once a slave fled to a non-slave state, it effectively diminished states' rights to protect the interests of its citizens (i.e. property interests in slaves).

The biggest reason why the ACLU takes the legal stance it does is because many of its supporters support a so-called right to be free of violent crime. In particular, that means the right of local communities suffering from criminal gun violence to pass laws restricting gun rights. Initially that was a round-about way of referring to black, inner-city communities, but with the increasing attention given to gun massacres in predominantly white, suburban, areas, that "right to be free of violent crime" has become more inconclusive.

It's important to understand as a legal matter the ACLU's stance effectively advances a states' rights argument, albeit in a narrow circumstance. However, their policy explanation for why they're doing that is couched in terms of what's effectively an individual right, specifically the right to be free of gun crime.

The policy justification is arguably consistent with the ACLU's usual focus on human rights matters, but also rather inconsistent from a legal perspective in that the concept of a right to be free of [gun] crime as an excuse to limit others freedom is quite tortured. The history regarding states' rights issues, and the perpetual discriminatory treatment of minorities--specifically black minorities--by states that traditionally defend gun rights adds several layers of complexity. That complexity is important and relevant, but it doesn't justify the tortured conceptual logic of the ACLU's policy reasoning.

From a purely textual and historical perspective I think it's pretty much beyond dispute that the 2nd Amendment was about preservation of states' powers to regulate (or not regulate) guns. Many state constitutions had clauses that specifically framed it as an individual right. So, yes, gun rights were often and widely seen as an individual right in some places. But the Federal constitution was almost exclusively focused on defining the limits of when and how federal legislative powers trumped state legislative powers. So while the federal government couldn't pass laws abridging political speech, states were free to do exactly that for 150 years. That's why the 1st Amendment isn't couched in terms of an individual right, but in terms of restricting federal powers--... shall make no law ... abridging the freedom of speech. That's because the real issue of the day was whether the federal government could pass laws that interfered with state laws protecting _or_ restricting behavior. It's only where the Federal government was expected to have a direct and necessary relationship with an individual person (eminent domain, search & seizure, trial) where things are framed as individual, inalienable rights.

This was especially important in matters of gun rights. Because the federal government had ultimate authority to command state militias, states were afraid the federal government would limit their ability to defend themselves. The right of a state to defend itself basically boils down to the ability to prevent and suppress insurrections and rebellions, especially but not exclusively slave rebellions. Controlling that meant states needed the freedom to both protect the gun rights of some people, but also to limit the gun rights of other people, and to generally regulate guns however they saw fit. Different states might have wildly differing laws based on their circumstance. (You can sort of see how that fits the ACLU's logic of freedom from crime, but the perspective is still different--securing a state's regulatory power vs focusing on a very specific, individual "right".

But after the Civil War, and in particular the passage of the 13th and 14th Amendments, there began a shift of using Federal constitutional laws to limit both federal and state powers, and thus a shift toward emphasizing particular individual rights. Remember, the inalienable rights Jefferson mentioned in the Declaration of Independence were vague--life, liberty, and happiness. The next sentence says that the way to secure those vague rights was by securing the ability of men to govern themselves. It's a defense of democracy. It doesn't proscribe rules for limiting government power; on the contrary it affirms the power and legitimacy of self government to contravene pre-existing laws and limitations in order to secure individual rights as they saw fit. And in the context of a federal government, the highest concern was preserving the power of those individual state governments to address liberty issues as they best saw fit.

The Civil War finally proved how limiting that conception of government was in actuality, even though it was long theoretically understood. It proved that even well-governed democracies were prone to act in a tyrannical manner even despite their own laws. (The 14th Amendment secures the right to equal protection of the laws.) The nation wasn't yet prepared to tackle that dilemma until it became an existential issue 80 years later. But the point is, you can't read the Federal constitution in terms of individual rights. Little of the original text makes sense from that perspective. But neither can you understand and explain subsequent legal and political reasoning, and the evolution of that reasoning, without understanding the complete history.