As police departments around the country report a surge in 3D-printed firearms turning up at crime scenes, gun safety advocates and law enforcement officials are warning that a new generation of untraceable weapons could soon eclipse the “ghost guns” that have already flooded U.S. streets.At a summit in New York City on Thursday, the advocacy group Everytown for Gun Safety will bring together policymakers, academics, 3D-printing industry leaders and law enforcement officials to confront the growing challenge. They fear that as the printers become cheaper and more sophisticated — and blueprints for gun parts spread rapidly online — the U.S....