WASHINGTON, Aug. 24 - A type of lightweight police vest used by tens of thousands of officers failed to stop a bullet in nearly 6 of every 10 tests, according to a Justice Department study released on Wednesday, and the study resulted in immediate changes in federal safety guidelines. Ballistic tests on 103 vests containing a fiber known as Zylon produced acceptable safety results for just four vests, department researchers said. "This confirms that these vests simply don't do what they claim to do, which is to stop bullets," said Ed Balzarini Jr., a lawyer from the Pittsburgh metropolitan region...