The field of firearms identification is typically associated with tool mark identification. In reality much of firearms identification entails a specific area of tool mark identification. By definition, a tool mark results from the contact of one surface with another, the harder of which is the tool. Thus, in the case of a firearm and a bullet, the firearm is the tool that produces tool marks on the surface of the bullet as it moves through the barrel upon discharge of the firearm. Likewise, the examination of firing pin impressions, magazine marks, extractor marks, ejector marks, breech face marks, and chamber marks on fired cartridge cases all constitute tool mark examinations.
Firearms identification also involves examinations other than tool marks, and these account for the distinction between the two disciplines. The analysis of gunpowder patterns on clothing, the determination of cartridge case ejection patterns, the measurement of trigger pull, or establishing bullet trajectory are examples of this discipline. Likewise, weapons function testing, shot pellet pattern testing, and serial number restoration are additional non-tool mark comparison aspects of firearms identification.
Firearms identification is often referred to as "forensic ballistics" or simply "ballistics," but this is a misnomer, as ballistics is limited to the study of projectile behavior. Ballistics, in the true sense, includes three
Suppose that a fired cartridge case is found at a crime scene. Also present is a 9 mm pistol. The firearms examiner uses the scientific method to evaluate these items of physical evidence.
Step 1: Define the problem. Was the cartridge case found at the scene fired from the 9 mm pistol? Step 2: Make observations.
■ The fired cartridge case has "9 mm Luger" stamped on the base.
■ The weapon is chambered for a 9 mm Luger.
■ The weapon is a Glock (manufactured in Austria by Glock Industries).
■ Glock 9 mm semiautomatic pistols produce rectangular firing pin impressions in the primers of cartridge cases they fire.
■ The cartridge case found at the scene has a clearly recognizable rectangular firing pin impression.
Step 3: Form a hypothesis or theory. The fired cartridge case found at the scene was fired in the Glock pistol also found at the scene. Step 4: Carry out tests. The fired cartridge case and the Glock pistol are taken back to the crime laboratory, where the pistol is test-fired and the fired cartridge cases produced are compared to the evidence cartridge case using a forensic comparison microscope. The results indicate that the evidence cartridge case has general features like those present on the test fires, but there are marks on the test fires that are not present on the evidence cartridge case and vice versa.
Step 5: Modify the hypothesis. The cartridge case found at the scene was not fired from the pistol also found at the scene. The pistol found at the scene does not appear to have been involved. Step 6: Develop a theory. A Glock pistol, or a weapon that produces similar marks on cartridge cases, was used to fire the cartridge case found at the scene.
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