When Gun Laws Don’t Prevent Gun Crime

On Monday, in the city of Highland Park, Ill., a deranged goblin of a man opened fire on a July 4 parade, killing seven innocent people and wounding three dozen others. After an intense search, the culprit was apprehended and taken into custody. Yet again, a mass shooting has sullied America.

And, yet again, it is unclear what lawmakers can do to prevent the next one. Just weeks ago, the Senate passed a gun-control bill that Chris Murphy described as “the most significant piece of anti-gun violence legislation in nearly 30 years.” Today,...