 The firearms identification unit of the FBI laboratory is primarily concerned with identifying the gun from which a particular bullet was fired. They also examine many other factors involving firearms, tool marks, and explosives in the commission of a crime. In this case, a spent bullet found in the victim, a cartridge case found at the crime scene, and a suspect weapon have all been received from a law enforcement agency for examination by the FBI laboratory. The examiner assigned to the case is making a preliminary examination of the bullet and cartridge case and marking both the containers and the evidence itself for identification throughout the examinations and possible court testimony. Another FBI expert is meanwhile examining the suspect weapon for fingerprints. Although none appear on the gun, he finds some on the cartridge clip and takes this with him for photography and classification of the fingerprints. The firearms examiner checks the weapon briefly for type, condition, and serial number. This pistol being of a common semi-automatic type and presenting no unusual features, he need not refer to the extensive firearms collection which includes both shoulder weapons and handguns. He is ready to test fire the weapon. At least two shots are fired into the box which is packed with cotton and batting to stop the test bullets without marking them. The examiner locates and carefully removes the test bullets from the box. Like the two ejected cartridge cases he brought with him from the firing point, they will bear the unmistakable markings of the weapon from which they were fired, the pistol from the suspect. He examines the markings under a microscope. The test cartridge cases show the impression of the firing pin and perhaps other marks made by the breech face, extractor, or ejector. The bullets are marked in a unique way by the gun barrel through which they have passed. Now with a comparison microscope, he can match these test cartridge cases and bullets with the evidence. He places the test cartridge case on one stage of the microscope, the cartridge case from the crime scene on the other stage. After adjustments of position and focus, he compares directly the marks appearing on one cartridge case with those on the other. Now he places the bullets on the two stages of this costly precision instrument. One of the test bullets he has just fired and the evidence bullet recovered from the body of the victim. He adjusts the focus, rotates one bullet until it is directly lined up with the other. The composite image shows the striations on the bullets made by passing through the gun barrel. Like fingerprints, these markings are unique. A match shows that both bullets were fired from the same weapon. Comparison of bullets and cartridge cases to arrive at such findings is a major activity in the firearms identification unit. But examiners use many other techniques in reconstructing the circumstances of a crime in which firearms are used. Expert examination of a shattered windshield may yield information on the direction of the fire, the order in which the bullet holes were made, and an estimate of the size of the projectile and the angle of fire. Other examiners may fire shotgun shells at known ranges to estimate the distance of a murder weapon from the victim. The size of the shot pattern can provide this information. Another kind of examination looks for powder residues around the bullet hole in the victim's clothing. If any can be found, it may be chemically analyzed to arrive at an estimate of the distance from which the shot was fired. This examiner is taking down an evidence gun which misfired during test. Behind him is another notable reference collection, the file drawers containing the standard ammunition file. To replace a defective spring of the evidence revolver, the examiner checks the gun against the reference collection of 1200 handguns. This collection, with its many rare weapons, is used for a variety of investigative purposes. The first examiner's investigation is finished. He can report that the weapon submitted in evidence and only it could have fired the bullet found in the victim's body and the cartridge case found at the scene. And he is prepared to back up these findings in court as an expert witness. These are the services the firearms identification unit of the FBI laboratory offer without charge to any law enforcement agency.