Whatever one might plausibly say about whether the Second Amendment secures an individual or collective right, and whether it applies to citizens in D.C., it seems to me one can't plausibly say (as the dissent in the D.C. Circuit case did) that the majority opinion is "dicta," or that "the meaning of the Second Amendment in the District of Columbia (District) is purely academic."

Dictum, or the plural dicta, refers to "An opinion by a court on a question that is directly involved, briefed, and argued by counsel, and even passed on by the court, but that is not essential to the decision" (Black's Law Dictionary). Dictum is not binding on future panels within the Circuit -- though it might be persuasive, just as concurrences (set aside concurrences in the judgment needed to form a majority) are not binding and thus pure dictum, but might be persuasive, and just as dissents might sometimes be persuasive (at least when the contrary view expressed by the majority is not binding). It is also sometimes seen as poor form, though my sense is that nearly all judges engage in it on occasion with little embarrassment.

Thus, if the majority had stated that it thought the right to bear arms was incorporated against the states, that would be dictum, because that conclusion would likely not be essential to the logic of the majority's decision: Since the majority is speaking of a right as against a federal entity under the Second Amendment (the District of Columbia), and its reasoning does not require a judgment about what the right would be as against a state under the Fourteenth Amendment, any opinion on the matter is "not essential to the decision" that the panel handed down.

But deciding whether the right is individual -- and surely deciding "the meaning of the Second Amendment in the District of Columbia" -- is essential to the majority's reasoning. The majority's ultimate conclusion, which is that the D.C. gun ban is unconstitutional as to the one plaintiff who has standing, relies on the intermediate conclusion that the right is individual, and therefore applies to individuals throughout the country, in states or out. As the majority points out on p. 48 n.16, it's possible that the majority's intermediate conclusion is wrong. But, right or wrong, it "directly supports [the majority's] holding," and is therefore holding, not dictum.

Finally, note that reasoning does not become dictum just because the majority could have reached the same result through a narrower decision (an argument that had been made as to the Fifth Circuit Emerson decision holding the Second Amendment to be an individual right, but holding that the Amendment wasn't violated by the statute at issue in that case). For instance, in Everson v. Board of Education (1947), the Court decided that the Establishment Clause applied to the states and not just to the federal government; and the Court also defined in some measure the scope of the Clause. Nonetheless, the Justices went on to apply this (admittedly quite ambiguous) definition to conclude that the Establishment Clause was not violated by the practice at issue in this case. The Court could have just said "even if the Establishment Clause applies to the states, the practice here would not violate the Clause." This would have reached the same result through a narrower decision. But it was not the reasoning that the Court used; and given that deciding on whether the Clause was incorporated was an essential part of the Court's reasoning, that decision is binding holding, not dictum. (For other examples, see here.)

But in any event, regardless of what you think about the "majority could have reached the same result through a narrower decision, so their broader decision is dictum" argument, it can't apply here: Here, adopting the dissent's supposedly narrower view would have yielded the opposite result. Again, the majority's reasoning may well be incorrect holding, but it's holding.