An organization that sets standards for business transactions has voted to create a special code for gun stores to use when processing credit and debit card transactions, a move long sought by gun safety advocates who say it will help law enforcement officials spot suspicious purchases. The decision by the group, the International Organization for Standardization, or ISO, is likely to set off a political firestorm in Washington and around the country as well as a debate among banks and companies about how and even whether to follow the new standard.