Chapter 9: Firearms, Tool Marks, and Other Impressions

Firearms, Tool Marks, and Other Impressions forensic science chapter summary explores the comprehensive methodologies used in firearms identification, tool mark analysis, and the examination of impression evidence, providing a crucial guide for understanding how ballistics and physical patterns link suspects to crime scenes. The text begins by categorizing firearms into handguns and long guns while explaining the mechanical process of rifling, where lands and grooves are bored into a barrel to improve accuracy. It distinguishes between class characteristics, such as caliber and direction of twist, and the individual characteristics formed by microscopic striations that allow experts to match a fired bullet to a specific weapon using a comparison microscope. The summary covers the analysis of cartridge cases, identifying unique signatures left by firing pins, breechfaces, ejectors, and extractors, and explains how these markings are cataloged in the National Integrated Ballistic Information Network (NIBIN). Detailed attention is given to distance determination, describing how gunpowder residue patterns, bullet wipe, and stellate tearing on targets help investigators estimate the range of fire, utilizing visualization methods like infrared photography and the Greiss test for nitrites. The detection of primer residues—specifically barium and antimony—on a shooter's hands is explained through the use of adhesive lifts and scanning electron microscopy (SEM). Additionally, the chapter outlines the chemical etching process used to restore obliterated serial numbers by targeting strained metal crystals. The discussion extends to tool marks, describing how abrasions and cuts are compared to suspect tools, and concludes with protocols for preserving footwear and tire impressions, including electrostatic lifting of dust prints, casting with dental stone or snow wax, and chemical enhancement of bloody impressions.