ABSTRACT
Before surveying the types of firearms currently being manufactured, some comments on the mechanisms of automatic and semiautomatic firearms are in order. Automatic and semiautomatic weapons function in one of three ways: blowback, recoil, or gas piston. In blowback weapons the fired cartridge pushes the breechblock backward against a spring; the expended cartridge is extracted by the moving breechblock and ejected from the weapon. The compressed spring then pushes the breechblock forward, removing a cartridge on the magazine and inserting
it into the firing chamber. In the recoil operating system, the barrel of the weapon and its breechblock recoil a short distance together; the breechblock then unlocks from the barrel and continues to recoil rearward against a spring. The compressed spring returns the breechblock to its original position, loading a fresh cartridge into the firing chamber. In gas-operated weapons, a small amount of the propellant gases passes through a small hole in the barrel into a gas piston. The expanding gas forces the piston to the rear; a rod connects the piston to the breechblock so that the breechblock is also pushed to the rear. The types of firearms currently available are summarized below. (For a discussion of career preparation for this field, see Sidebar 14.1.)
There are two types of handguns. In a revolver, the cartridges are held in firing chambers in a rotating cylinder. Single-action revolvers are fired by manually cocking the hammer and then pulling the trigger. Cocking the hammer both rotates the cylinder to place one of the chambers under the hammer and cocks the firing mechanism. Double-action revolvers are fired by a long trigger pull that raises the hammer, indexes a firing chamber under the hammer, and then allows the hammer to drop, firing the cartridge. The other type of handgun is a semiautomatic pistol. These weapons are also referred to as autoloaders or self-loaders; laypersons sometimes incorrectly refer to these weapons as automatic pistols. Figure 14.1 shows a Beretta semiautomatic pistol. In semiautomatic pistols, the cartridges are held in a
SIDEBAR 14.1. CAREER PREPARATION AND EXPECTATIONS
Preparation for a career in the examination of firearms and tool marks is generally similar to that described in the previous chapter for fingerprints. In the past, law enforcement officers typically made the transition to the laboratory through apprenticeship with experienced examiners. Now, most positions require a college degree in a natural science for an entry-level position. A long training period and apprenticeship are still required, often accompanied by intensive training courses offered by agencies such as the U.S. Bureau of Alcohol, Tobacco, Firearms, and Explosives (www.atf.gov). Once on the job, examiners spend most of their time working in a forensic laboratory environment doing cases and continuing education.
