Last month, a federal judge ruled that New Jersey’s ban on AR-15 rifles is unconstitutional. A week later, a federal appeals court deemed a similar ban in Maryland perfectly consistent with the Second Amendment. These dueling decisions reflect a basic disagreement about whether the Second Amendment allows the government to ban guns that are commonly used for lawful purposes, as opposed to “dangerous and unusual weapons.” The answer seems clear based on the Supreme Court’s precedents. The Court’s landmark 2008 decision in District of Columbia v. Heller, which overturned a local handgun ban, not…