I think you are confused on what these terms mean. "Collective right" obviously does not mean that that individuals cannot own guns. It means that the right to own guns is in the context of a regulated militia. Obvious regulations that a militia might want to have are minimum age requirements, training requirements, equipment safety requirements (such as requiring owners to store and transport their guns safely and perform regular maintenance). "Collective right" means that the right isn't an individual right, in other words, individuals do not have the right to buy, sell, and own any firearm they want in any condition they want for any purpose they want.
These definitions are abundantly clear in any good-faith conversation about gun policy. The ACLU in particular is quite clear about which policies they believe infringe on civil liberties and which do not. For instance, the ACLU's position is that laws which restrict the types of weapons or ammunitions which can be owned generally do not infringe on civil liberties, while laws which restrict categories of people from owning guns often do infringe on civil rights (the examples they give are immigrants and people with mental disabilities). Incidentally, the ACLU also believes that many current federal restrictions on gun ownership do infringe on civil liberties, like the prohibition of certain convicted criminals and people with mental disabilities when there is no indication of violent behavior.
> It means that the right to own guns is in the context of a regulated militia.
Ok this is already an oddity. There's no second such right. I mean, the right for religious freedom does not require you to join a regulated religion (even if many do join organized religions). The right for free speech does not require you to be a journalist. The right for speedy trial does not require you to join a speedy trial club and pass regular legal training. And so on. This is what I'd call odd. But there's more.
> Obvious regulations that a militia might want to have are minimum age requirements, training requirements, equipment safety requirements (such as requiring owners to store and transport their guns safely and perform regular maintenance).
OK let's assume so. Note that most gun regulations in anti-gun states and cities have little to do with this - but let's assume that's what ACLU wants.
> For instance, the ACLU's position is that laws which restrict the types of weapons or ammunitions which can be owned generally do not infringe on civil liberties
How this follows? How "you can not own a gun with certain feature" fits with the above? Does this feature prevent training, or is particularly unsafe, or preclude safe transportation and storage? Not at the least. So how does it fit in this concept? It comes to the comical, that two models of the same gun, with small cosmetic change, may one be legal and another illegal in certain states (OK, let's not be coy, in California) - can anybody seriously argue that's because one can be used in militia and another can't? That's beyond odd, that makes no sense. I mean if we talked about guns that somehow useless to militia (don't know, let's assume you found an argument that specific type of a gun is so peculiar that no militia would ever have a reason to use it) - ok, I'm willing to accept that makes sense in that framework. But that's not how the regulations that ACLU supports and endorses work!
> while laws which restrict categories of people from owning guns often do infringe on civil rights
Again, how does it work? You named age restriction yourself. Isn't it "category of people"? But let's say we talk about people who are violently insane, or that are career felons, maybe violent gangsters who were imprisoned for many years for their crimes. You don't want such guys in your militia, probably, right? If you have them there, they'd probably loot everything instead of defending the fellow citizens, maybe? Yet it's absolutely "category of people". Let's take mental disabilities - of course, there is a whole gamut of disabilities, but in general you'd want your militia to be mentally stable, right? So how does it work? It looks again that the position does not even have internal consistency, that's even before anybody would start to attack it on merits of its principles! Again, that's a very odd position to hold.
These definitions are abundantly clear in any good-faith conversation about gun policy. The ACLU in particular is quite clear about which policies they believe infringe on civil liberties and which do not. For instance, the ACLU's position is that laws which restrict the types of weapons or ammunitions which can be owned generally do not infringe on civil liberties, while laws which restrict categories of people from owning guns often do infringe on civil rights (the examples they give are immigrants and people with mental disabilities). Incidentally, the ACLU also believes that many current federal restrictions on gun ownership do infringe on civil liberties, like the prohibition of certain convicted criminals and people with mental disabilities when there is no indication of violent behavior.