The ACLU uses the usual leftist misreading of Miller as the basis for its policy:
Gun Control
Updated: 1/17/2013
The Second Amendment provides: "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
ACLU POSITION
Given the reference to "a well regulated Militia" and "the security of a free State," the ACLU has long taken the position that the Second Amendment protects a collective right rather than an individual right. For seven decades, the Supreme Court's 1939 decision in United States v. Miller was widely understood to have endorsed that view. This position is currently under review and is being updated by the ACLU National Board in light of the U.S. Supreme Court decision in D.C. v. Heller in 2008.
In striking down Washington D.C.'s handgun ban by a 5-4 vote, the Supreme Court's decision in D.C. v. Heller held for the first time that the Second Amendment protects an individual's right to keep and bear arms, whether or not associated with a state militia. The ACLU disagrees with the Supreme Court's conclusion about the nature of the right protected by the Second Amendment. However, particular federal or state laws on licensing, registration, prohibition, or other regulation of the manufacture, shipment, sale, purchase or possession of guns may raise civil liberties questions.
ANALYSIS
Although ACLU policy cites the Supreme Court's decision in U.S. v. Miller as support for our position on the Second Amendment, our policy was never dependent on Miller. Rather, like all ACLU policies, it reflects the ACLU's own understanding of the Constitution and civil liberties.
Heller takes a different approach than the ACLU has advocated. At the same time, it leaves many unresolved questions, including what firearms are protected by the Second Amendment, what regulations (short of an outright ban) may be upheld, and how that determination will be made.
Those questions will, presumably, be answered over time.
In Miller it is clear that SCOTUS intended to protect arms that have "reasonable relationship to the preservation or efficiency of a well regulated militia."
And the militia "comprised all males physically capable of acting in concert for the common defense. 'A body of citizens enrolled for military discipline.' And further, that ordinarily when called for service these men were expected to appear bearing arms supplied by themselves and of the kind in common use at the time." Nowhere does Miller say that a person must be enrolled in, active in, or trained by the militia.
