READ: Arson, Firearms and Toolmarks

Properties of Firearms

A firearm is a weapon, especially a pistol or rifle, capable of firing a projectile and using an explosive charge as a propellant. Essentially, this means that all firearms must have a charge material such as gunpowder, a projectile such as a bullet and a barrel to fire from. 

Firearm Anatomy

There are many different types of firearms; this module will focus on the differences between barrels that have been rifled, such as rifles and handguns, and shotguns which have not been rifled. Some of the most common types of guns found are rifles, pistols, revolvers and shotguns. A rifle is a shoulder firearm with a rifled barrel designed to fire one projectile, or bullet, at a time. A pistol is firearm designed to be held and fired with one hand. A revolver is a type of pistol having a revolving cylinder with several cartridge chambers that may be fired in succession. A shotgun is a firearm with a smoothbore, or smooth barrel without rifling, designed to fire small pellets called shot or rifled slugs. A different set of guns known as semi-automatic and fully automatic guns, are somewhat less common. A semiautomatic weapon fires one round with each individual trigger pull without the need to reload or reset the gun after each shot. A fully automatic weapon continuously fires rounds while the trigger is pressed and held. These weapons use a special type of ammunition held together so that they can be fired in rapid succession called a magazine. A magazine is the part of a repeating firearm which holds the cartridges or shells in position ready to be loaded one at a time into the chamber.

 Some of the major parts of a gun include the barrel, muzzle, breech, trigger, and hammer. The barrel is the metal tube of a firearm made from iron or steel, through which the bullet or shot charge passes when the firearm is fired, whereas the bore is the tunnel down the barrel of a firearm through which the projectiles travel. Gun barrels can be rifled or smooth; rifles and handguns have rifling whereas shotguns have smooth barrels. Rifling is the spiral grooves cut into the inside barrel surface to cause a bullet to spin, thereby stabilizing it. The cut-away portions of the rifling are called grooves and the uncut portions are called lands. The rifling causes the bullet to spin before it leaves the barrel, resulting in greater accuracy and range. Lands are the raised areas between two grooves in the rifling of a gun barrel that impart grooves on the bullet. Grooves are spiral cuts into the bore of a barrel that give the bullet its spin or rotation as it moves down the barrel. They look like depressions in the bore next to the raised lands. Caliber is the diameter of the bore of a rifle before the rifling grooves are cut. The rifling is used to increase both accuracy and range on a firearm. When Muskets had smooth bores, their range was only 50-75 yards with fairly poor accuracy. With rifling, Musket range increased to about 200-300 yards and accuracy also increased!

The muzzle is the forward end of a barrel. Historically, many of the earliest firearms were muzzle loaded which meant that the the ammunition was loaded from the front end of the barrel with a propellant such as gunpowder. While this method worked most of the time, it was time-consuming to load and did not allow for more than one shot per loading. It also was prone to misfires and led to more dangerous conditions.

The breech is the portion of the gun that contains the firing mechanism which is generally located at the back end of the barrel. Bullet cartridges are loaded into the breech end of the gun for firing. A cartridge is a case, usually made of brass or copper, containing the power charge, the primer and the bullet. It is also known as a shell casing. The firing pin is the part of the breech mechanism which strikes the primer of the cartridge. When a firing pin strikes the primer of the cartridge, it makes a unique marking on the back of the cartridge that can be examined to verify matching bullet cartridges. Interestingly, a method was developed in which the firing pins of modern firearms could be engraved by laser with microscopic markings or serial numbers so that they could be identified in Forensic investigations. While the method is still being refined, it could someday become commonplace to have each cartridge discharged with a unique serial number that could be used to link to a specific gun. The firing pin in a firearm is struck by a structure known as the hammer. The hammer is the part of a gun that hits the primer or firing pin, or explodes the percussion cap, and causes the gun to fire. The hammer is activated by pressing the trigger. The trigger of a gun is defined as the lever that activates the firing mechanism of a gun.

The "Sawed-off Shotgun" is a shotgun in which the barrel has literally been sawn off to 18 inches or less. This is done for one or more of the following reasons: to make the weapon easier to conceal, to make it easier to shoot one-handed and to force the shot to spread in a wider pattern when it exits the gun. While they these types of shortened shotguns are legal with special permits, they are of interest in Forensic investigations because of they are often used to commit crimes.

How a Firearm Works

Initially a cartridge enters the chamber and the firing pin is held back due to a spring. When the trigger is pulled, the spring releases sending the firing pin forward. The firing pin then hits the base of the cartridge, igniting the primer powder which sparks through the flash hole to the gunpowder. The gunpowder is the main propellant supply. The pressure of the exploding propellant pushes the bullet from the casing into the barrel. The bullet follows the lands and grooves produced by rifling and spirals out of the barrel. The casing is then ejected and a new bullet can enter the chamber.

Georgia Virtual, Firearms, Ballistics, Toolmarks, and Arson, CC BY-NC-SA 3.0