MEXICO CITY — The United States should reinstate a Clinton-era ban on assault weapons to prevent such guns from reaching Mexican drug cartels, former officials from both countries said in a report released Tuesday. The group, which includes two former U.S. ambassadors to Mexico, also said the U.S. should do more to stop the smuggling of firearms and ammunition into Mexico by stepping up investigations of gun dealers and more strictly regulating gun shows. The Binational Task Force on the United States-Mexico Border listed the assault weapons ban as a step the U.S. should take immediately to improve security in...