A landmark 2016 law praised as an ‘unbelievably powerful tool’ against gun violence remains scarcely used

Two weeks after a gunman in Parkland, Fla., zigzagged from one classroom to another in his former high school, killing 17 people, a Beverly Hills High employee dialed 911 to report a stranger mumbling to himself and following her as she left campus.

Police confiscated a loaded handgun in the man’s…