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Comparing used bullet cartridges. Forensic scientist using a microscope to compare markings on the back of handgun cartridge shells. The shells held bullets, but were ejected from a gun as the bullet was fired. Unique markings are made as the firing pin of a gun hits the 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 (seen on the screen) can also be kept in a database and used if the gun is recovered at a later date. (KEYSTONE/SCIENCE PHOTO LIBRARY/MAURO FERMARIELLO)