|Ballistics, in criminal investigation, is the study of firearms and bullets. Firearms have been with us for several centuries – the first handguns were used by Arabs around A.D. 1200 – and as early as the 16th century, engineers realized that a spiral groove etched into the gun barrel would impart a spin to the projectile, thus making its flight more stable and improving its aim. It is the groove, or rifling, that leaves the distinctive marks known as striations on the bullet and that forms the bedrock of modern ballistics study.
The barrel is not alone in leaving identifiable traces on a cartridge; other gun components may also leave marks. When fired, the bullet is driven forward through the barrel; simultaneously, its shell casing hurtles back against the breech face. Any imperfection on that breech face impresses itself on the case head. The firing pin, the extractor and the ejector post may each etch marks on the head or shell casing.
The modern cartridge was invented in France in 1835 and consists of a casing with a soft metal cap holding the primer charge. When struck by the gun’s firing pin, the primer ignites the main propellant charge, expelling the bullet from the gun and leaving the case behind. With the invention of smokeless powder at the beginning of the 20th century came the need for a stronger bullet. The new powder’s greater propellant velocity meant that earlier lead bullets were too soft to be gripped by the rifling in the barrel and tended to get stripped, fouling the barrel. This led to the introduction of the metal-jacketed bullet, usually made of cupronickel.
Whereas the old black powder left distinctive marks on the hands of the firer and around the wound (if fired within a close enough range), modern smokeless powder leaves traces that can be detected only through chemical testing. At one time, suspects’ hands were examined to see if they had fired a weapon recently. The test was designed to detect the presence of nitrates, but because of the prevalence of nitrates in so many innocuous household products, this practice has largely been abandoned.
Culled from: The Casebook of Forensic Detection