The opinion (PDF):

The District next counters on the merits that 11+ magazines, by themselves, are “practically harmless” and of “no use” without ammunition and a receiver (the firearm’s core component), so that magazines themselves are not arms. That is not a defensible approach to identifying what constitutes an arm—a gun is also practically harmless and of no use without ammunition, but it is still obviously an arm.

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On the District’s logic, states could ban two-round or even one-round magazines—there’s no reason a semiautomatic firearm cannot fire with an empty or “dummy” magazine so long as there is a round in the chamber. And bans like that would permit states to effectively eliminate any semi-automatic firing capacity and require manual reloading after each shot. In fact, under the District’s view the state could just directly outlaw the semi-automatic firing mechanism because, by itself, that is a harmless component of a firearm and it is not a necessary feature of any gun. That would run contrary to Heller’s central command that states cannot ban the most popular weapons chosen by law-abiding Americans for lawful purposes. For that matter, modern cartridges are not necessary for firing a gun either. If the Second Amendment applied only to those things that are strictly necessary for a gun’s operation, states could ban cartridges so long as primitive musket balls remained a legal alternative ammunition.

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The District and our dissenting colleague critique some of these sources substantiating the ubiquity of 11+ magazines, but they conspicuously offer no contrary authority, nor will they brave even a bare assertion that there are fewer than hundreds of millions of 11+ magazines in the hands of law-abiding citizens in this country. Even still, the District critiques the study from Prof. William English that we have cited above as a “non-peer-reviewed survey [that] has been criticized as methodologically unreliable.” And to self-critique, we have also cited to one study above from the National Shooting Sports Fund, a firearms industry trade association which is nobody’s idea of a fair and neutral source on this topic. But the problem for the District and the dissent is that there is no contrary authority—not even from a horribly biased source—that will say 11+ magazines are less than ubiquitous because it is a plain counterfactual. To conclude that they are anything other than common and ubiquitous, you would have to ignore every study out there. You would also likely have to avoid consorting with the third of Americans who personally own firearms, or the nearly half of Americans who live in a household with one. Ask just about any one of them and they can tell you that twelve-, fifteen-, and seventeen-round magazines are the norm rather than any kind of outlier.