A first-of-its-kind law in New York could force 3D printers sold for homes and business to come equipped with technology blocking them from making guns. The new requirement, also under consideration in California, attempts to thwart the latest technique for producing untraceable “ghost guns” that have turned up in crimes. But there are questions about whether the technology can work and concerns about its effect on personal privacy and constitutional rights. About one-third of U. S. states...