A government-sponsored study issued Wednesday recommends against creating a US firearm database, saying the science behind the proposed ballistics database is too murky. The idea is based on the assumption that each gun leaves unique markings on bullets, like fingerprints. Some say every new gun should be test-fired and have its markings entered into a database. Investigators could then use the database to identify which gun fired shell casings and bullets found at crime scenes. The 300-page report says it is unclear how unique those ballistic fingerprints really are, and that it might be too expensive and time-consuming to be worthwhile.