The central meaning of the Second Amendment has long been settled in the courts. The last time the Supreme Court directly addressed the provision -- which reads, "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" -- was in 1939, in a case called United States v. Miller. The court said that the Second Amendment's "obvious purpose" is to ensure the effectiveness and continuation of state military forces (the militia mentioned in the amendment), not to provide a private right to own...