That's U.S. v. Arzberger. The gun control law is the part of 18 U.S.C. § 3142(c)(1)(B) that requires that when someone is charged with possessing child pornography (among other crimes) and is freed on bail, he be ordered not to possess any firearm. Here's the discussion by Magistrate Judge James C. Francis IV (of the Southern District of New York) (some paragraph breaks added):

A year ago, I might well have taken for granted the authority of Congress to require that a person charged with a crime be prohibited from possessing a firearm as a condition of pretrial release.... [But, given D.C. v. Heller, t]o the extent ... that the Second Amendment creates an individual right to possess a firearm unrelated to any military purpose, it also establishes a protectible liberty interest [for Due Process Clause purposes]. And, although the Supreme Court has indicated that this privilege may be withdrawn from some groups of persons such as convicted felons, there is no basis for categorically depriving persons who are merely accused of certain crimes of the right to legal possession of a firearm.

Again, the next step in the analysis is to apply the Mathews v. Eldridge balancing test. The private interest at stake is paramount: the right to possess a firearm is constitutionally protected. In Heller, the Court made clear that there is no hierarchy of constitutional rights: "[t]he very enumeration of the right takes out of the hands of government — even the Third Branch of Government — the power to decide on a case-by-case basis whether the right is really worth insisting upon." The remaining Mathews factors play out much as they did in the analysis of the curfew requirement.

First, there is a serious risk that, in the absence of an individualized determination, an accused person will wrongly be deprived of his Second Amendment rights. Indeed, the Government may well find it difficult to articulate a nexus between an accusation of receiving child pornography and the need to prohibit possession of a firearm. Second, providing the defendant with an opportunity to be heard with respect to the appropriateness of this condition would reduce the potential error rate without creating a significant burden. And, finally, the Government's interest in ensuring the safety of the community would not be undermined by requiring an independent judicial determination of the danger caused by the defendant and the efficacy of the proposed bail condition.

Accordingly, the Adam Walsh Amendments [the name of the statute involved here -EV] violate due process by requiring that, as a condition of release on bail, an accused person be required to surrender his Second Amendment right to possess a firearm without giving that person an opportunity to contest whether such a condition is reasonably necessary in his case to secure the safety of the community. Because the Amendments do not permit an individualized determination, they are unconstitutional on their face. The Government's application to impose as a condition of bail that Mr. Arzberger not possess a firearm is therefore denied....

[Footnote:] The determination that the mandatory imposition of each of the conditions requested by the Government violates procedural due process does not preclude the Government from renewing its application provided that it is prepared to proffer a basis for requiring any or all of these conditions in Mr. Arzberger's particular circumstances.

This is formally a Due Process Clause holding, but it rests on a conclusion about the Second Amendment. Recall that the Due Process Clause (sometimes) entitles people to hearings only when there's a legally relevant factual dispute to be resolved. Persons "who assert a right to a hearing under the Due Process Clause must show that the facts they seek to establish in that hearing are relevant under the statutory scheme," or are made relevant by the Constitution.

If Congress were constitutionally allowed to ban all gun possession by people who have been indicted for possessing child pornography, whether or not those people are found to pose a special risk of gun violence, then no hearing would be required. That's why the excerpt I quoted begins with the sentence "A year ago, I might well have taken for granted the authority of Congress to require that a person charged with a crime be prohibited from possessing a firearm as a condition of pretrial release." If Congress could categorically do this, then there'd be no factual issue to resolve at the hearing. All the evidence in the world that this person is unlikely to pose a danger of gun violence will be irrelevant under the statutory/constitutional scheme.

The hearing is required only because at least some such indictees still have the "constitutionally protected" "right to possess a firearm" — presumably those indictees as to whom "such a [no firearms] condition is reasonably necessary in [their] case[s] to secure the safety of the community," which is to say those who don't pose any special risk of misusing their guns. (I say "special risk" because obviously just the general risk that any gun owner may misuse his guns can't be enough: If it were, no hearing would be needed, since such a risk is present for everyone.)

In any case, this is an important decision, though just from a magistrate judge. For other cases in which the Second Amendment has been relied on in a claimant's favor, see here, here, and here. But none of these cases involved a holding that a gun control law was unconstitutional, even as applied to a particular claimant.

For other views on gun possession by indictees, see State v. Winkelman, 442 N.E.2d 811 (Ohio Ct. App. 1981) (upholding a categorical ban on gun possession by indictees, though noting that it imposes only a "temporary limitation," with provision for relief "[s]hould the temporary limitation work an undue hardship upon the indicted party"), overruled on other grounds, State v. Frederick, 1989 WL 80493 (Ohio Ct. App.); State v. In, 18 P.3d 500, 503 (Utah. Ct. App. 2000) (also stating that such a ban is constitutional, but without a detailed explanation). Compare State v. Spiers, 79 P.3d 30, 34-35 (Wash. Ct. App. 2003) (ambiguous on whether a categorical ban on gun possession by certain kinds of indictees was constitutional, or on whether such a ban was constitutional when there was a finding that the indictee "poses substantial danger").