Firearms
A firearm is a weapon of attack or defense that expels a projectile via the action of the force exerted by the gases resulting from the rapid combustion of an explosive mixture. A firearm is often associated with the commission of a violent crime and is commonly found at crime scenes or on suspects. Also, many people who commit suicide use a firearm. An interest in firearms in forensic sciences is therefore, paramount. In criminalistics, the study of firearms consists first in the knowledge and identification of firearms and their ammunition, second in the internal, external, and terminal ballistics, and finally in the analysis of powders, primers, and their residues.
The birth and evolution of firearms is directly linked to the discovery of black powder. It is believed that the discovery of black powder dates from 1242, when the French monk Roger Bacon (1214–1294) wrote a letter describing the recipe for black powder. At that time, it was composed of about 40% saltpeter (potassium nitrate), 30% charcoal, and 30% sulfur. The first barrels, ancestors of the modern firearms, were developed at the beginning of the fourteenth century. At that time, the barrel was loaded from its end (muzzle), first with powder, and then with the projectile. The powder was ignited with a match, which was connected to the powder through the base of the barrel. Around 1800, mercury fulminate started to be used and the first primers were developed. In 1835, French arms manufacturer Casimir Lefaucheux (1802–1852) invented the first metallic cartridge. One
year later, in 1836, American arms manufacturer Samuel Colt (1814–1862) invented the revolver. The pistol was invented prior to that time, however, it was loaded by the end of the barrel. The modern semi-automatic pistol (using a magazine) was invented after the revolver in 1893.
A firearm expels a projectile at high velocity. The projectile is part of the cartridge. The cartridge consists of a shell holding the primer at one end and the projectile on the other with powder in the middle. The cartridge is inserted either manually or automatically in the barrel of the firearm. The trigger of the firearm is then pulled, which arms the hammer. At some point, the hammer is released and hits the firing pin, which hits the primer. The shock to the primer starts its combustion, which, in turns, ignites the powder in the cartridge. The powder combusts very rapidly and produces gases, which increase the pressure inside the cartridge (and therefore the barrel) tremendously. This pressure is in the order of 2,000–4,000 atmospheres. This pressure is exerted on the base of the projectile, which is pushed into the barrel. The projectile then exits the barrel at high velocity, usually ranging from 250–1,000 meters per second (273–1094 yards per second).
Firearms are classified in two main categories: light and heavy firearms. Light firearms include handguns and shoulder guns. Handguns are then further classified into revolvers, pistols (semi-automatic, automatic, and machine), and Derringers (single-shot and double-barreled pistols). Shoulder weapons are divided into two subcategories: weapons with a rifled barrel, such as rifles and carbines, and weapons with a smoothbore barrel, such as shotguns. It is important to understand that some shoulder weapons may have more than one barrel. They can have two or more one-over-the-other barrels or side-by-side barrels. There are some shoulder weapons that have a combination of rifled and non-rifled barrels. Among the rifled shoulder weapons are the semi-automatic and automatic assault rifles and machine guns. Usually, heavy weaponry includes weapons that shoot calibers above 12.7 millimeter and are found on vehicles or armored tanks. These are specialized, usually military, weapons and are not encountered in the daily routine of a crime scene unit. Finally, there is the category of improvised or homemade weapons, which includes an enormous variety of different weapons of all calibers and functions.
Firearms are characterized by many variables, such as brand, model, size, length of barrel, shape, color, and functionality. Some of the most important variables of the firearm are the general rifling characteristics (when
dealing with a rifled barrel), which include the caliber, the direction and degree of twist, and the number and width of grooves and lands. The caliber is correlated to the barrel's diameter and the power of the cartridges for which the firearm is designed. With few exceptions, a firearm is designed to use one given caliber. Upon shooting a projectile, the firearm leaves impressions on the projectile and the cartridge's casing. The observation of these impressions allows the forensic scientist to establish a link between the firearm and the elements of ammunition.
When dealing with a firearm found at a crime scene or on a suspect, the first security measure is to consider it as loaded and ready to shoot. Security with firearms is paramount and must be prioritized over everything else. If the firearm has just been found at a crime scene, it is possible to sketch, photograph, and take notes about it before touching it. Then, it is either placed in a container specifically designed to transport firearms and resist accidental discharges, or it needs to be secured. The firearm is then transported to the forensic laboratory where the firearms and toolmarks examiner can examine it.
