Analyzing bullet markings in order to trace guns used in multiple crimes, such as gang activities, is possible because a gun's firing pin, rifling, breech face and ejection mechanism can impart unique markings, called "signatures" on fired bullets or ejected casings. Comparison (dual) optical microscopes allow forensic experts to compare signature features as small as a micrometer or one fortieth the width of a human hair.
Digital imaging systems are used to gather and store bullet signatures in forensic databases, and it's important for these systems to record the best possible images. Crime labs can test the quality of their imaging systems by taking pictures of NIST's Standard Bullet and evaluating how well their system's analysis of the bullet's signature matches up with a certified signature provided by the Institute.
To make the Standard Bullet, NIST traced surface topography profiles on six master bullets fired by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) and the Federal Bureau of Investigation (FBI). Researchers fed the data to a computer-controlled diamond turning machine to engrave six typical bullet signatures on the standard. After machining, a specially designed chemical etching process was used to roughen the surface of the bullet to make it appear like a real bullet when observed under an optical microscope.
Earlier efforts at standard bullets produced by real guns could show minute changes in signatures due to gun wear and environmental conditions. However, since the signatures and dimensions of the NIST Standard Bullet are recorded for manufacture, subsequent copies are essentially identical. The etched and corrosion-protected NIST Standard Bullet is certified for use until June 30, 2016. In 2007, NIST hopes to produce a complementary standard for ejected casings by a different fabrication procedure. The bullet and casing reference standards will support the National Integrated Ballistics Information Network (NIBIN) goal of establishing nationwide ballistics traceability, measurement unification and quality control. NIBIN is a joint project of the FBI and the ATF.
Source: National Institute of Standards and Technology
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