MODEL RELEASED. Comparing bullets. Forensic scientist using a microscope to compare markings on bullets. The barrel of gun has grooves on the inside that make a bullet spin to improve accuracy when firing. These grooves leave marks on bullets fired from the gun, and are unique to each gun. If a bullet can be recovered from a crime scene, the markings can be analysed and compared to markings on bullets test-fired from suspect guns. If these markings match, then the gun was used to fire that bullet. The markings (seen on the screen) can also be kept on file in a database (this one is called Bulletproof) and used if the gun is recovered at a later date.