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Used bullet cartridge examination. Cartridge shell being placed in a microscope holder. Another shell (not seen) is in a similar holder for comparison. The shells held bullets, but were ejected from a handgun once the bullet was fired. Unique markings are made as the firing pin of a gun hits the back of a shell and as the shell recoils from the bullet. If a shell can be recovered from a crime scene, the markings can be analysed and compared to markings on shells test-fired from suspect guns. If these markings match, then the shell came from that gun. The markings can be recorded and kept in a database, and used if the gun is recovered at a later date. (KEYSTONE/SCIENCE PHOTO LIBRARY/MAURO FERMARIELLO) === ===