This week, a federal judge ruled that a major provision of New Jersey’s “assault weapon” ban is unconstitutional, but he was not happy about saying so. The decision illustrates how the Supreme Court’s Second Amendment precedents have constrained the discretion of judges who are personally inclined to support gun control. New Jersey’s Assault Firearms Law—which the state Legislature approved in 1990, responding to a mass shooting at a Stockton, California, elementary school the previous year—bans a list of specific rifle models, along with “any firearm manufactured under any designation which i…