It is probable that the word firearm originated from the flame produced at the muzzle end (muzzle flash) when a firearm is discharged; the muzzle is the front part where the bullet emerges from the barrel (Photograph 1.1).

The word gun is a widely accepted alternative name to firearm, although it is also used in many nonfirearm terms, for example, grease gun, spray gun, flame gun, nail gun, insecticide gun, paint gun, stun gun, etc. [A stun gun is

Photograph 1.1 Muzzle flash.

a weapon designed to disable a victim temporarily by delivering a nonlethal high-voltage electric shock. The gun needs to be in contact with the victim. A taser gun works in the same way but can be used at a distance, up to about 15 feet. It shoots small electrodes at the victim, thereby connecting the gun and victim through metal wires; 50,000 volts travel through the wires for 5 seconds. The gun has 18 watts of power output, which yields a very low amperage (0.00036 amps). Because amperage is very low, no serious or permanent injury is caused]. Heavy, large-caliber guns, as used in land/sea warfare, come under the category of artillery and are beyond the scope of this text. Small arms are firearms which can be carried by an individual.

Ballistics is the science of the performance of projectiles, relating to their trajectory, energy, velocity, range, penetration, and so forth. Exterior ballistics is concerned with the flight of the bullet after it leaves the muzzle of the gun. Interior ballistics is concerned with the primer ignition, the burning of the propellant powder, and the resulting internal pressures and torques as the bullet is forced through the barrel. Terminal ballistics is the study of the interaction of the projectile with the target.

A firearm is a tool designed to discharge lethal projectiles from a barrel toward selected targets. It is the means of aiming and discharging the projectile and imparting stability to it. In Northern Ireland the law defines a firearm as "a lethal barreled weapon of any description, from which any shot, bullet or other missile can be discharged."1 This very broad definition does not mention the means of causing the shot, bullet, or other missile to be discharged, but this may be by compressed air, by gas (for example, carbon dioxide cylinders), by mechanical means (for example, a spring), or by the rapid burning of a propellant (gunpowder). Because a firearm operated by the burning of a propellant is by far the most common and potentially the most lethal, only this will be considered here. (Anything used to resemble a firearm in the commission of a crime may be treated as a real firearm in a criminal law trial.)

Firearms can have different features such as the ability to carry more than one cartridge, to load and unload automatically, and to fire repeatedly on a single pressing of the trigger. These are refinements on the basic design. In its simplest form a firearm could be a crude metal tube with one end packed with some form of propellant which on ignition produces enough gas pressure to discharge a projectile or projectiles with sufficient energy to cause human death. In its most complicated form it might be a well-made and finely engineered tool capable of discharging and directing bullets on automatic fire up to a rate in the region of 1,500 rounds per minute over an accurate range of about 200 meters (1,500 rounds per minute is extreme, 600 to 800 rounds per minute is much more common and practical). Alternatively, it might be a high-powered, highly accurate sniper rifle equipped with telescopic sights and capable of killing a selected target at a distance of 1,000 meters or more.

Firearms are relatively cheap, readily produced, reliable, and deadly and find many uses, among which are warfare, sport, self-defense, hunting, law enforcement, and crime. It is the use of firearms in crime that demands the attention of the forensic scientist.

The most commonly used firearms in crime are pistols, revolvers, and rifles up to and including .455" caliber, and shotguns, the most popular of which is the "sawn-off"12-bore caliber (Photograph 1.2 through Photograph 1.7 illustrate different types of firearms). This discussion deals with these weapons although submachine guns; machine guns; larger-caliber firearms; homemade firearms; air, spring, and gas guns; imitation and replica firearms are also encountered in crime, but to a much lesser extent. (A submachine gun is a lightweight machine gun that is hand held as compared to a machine gun that is bipod/tripod mounted for continuous fire.)

Pistols and revolvers are usually described as handguns, and rifles and shotguns as shoulder guns, as this is their normal mode of use.

A revolver is a type of pistol but for clarity it is better to describe it separately. A revolver is a single-barreled handgun with a revolving cylinder (multiple chambers), which holds a number of rounds of ammunition

Photograph 1.2 Machine gun.

Photograph 1.3 Submachine gun.

Photograph 1.4 Pistol.

Photograph 1.5 Revolver.

Photograph 1.6 Shotgun.

Photograph 1.7 Rifle.

(usually six). Each time the trigger is pulled, the cylinder is mechanically rotated so that each successive round of ammunition is placed in the firing position, that is, in line with the barrel. The spent cartridge cases are not ejected automatically but have to be removed manually.

A pistol is a single-barreled handgun in which the chamber is an integral part of the rear end of the barrel. A pistol can be either the single shot type, with manual or automatic ejection of the spent cartridge case, or much more commonly the self-loading type. In self-loading pistols a number of rounds of ammunition are loaded into a magazine, which is usually fitted into the handgrip of the weapon. Once the weapon is initially cocked and discharged a reloading mechanism, which is operated by the force of recoil or by gas pressure, extracts and ejects the spent cartridge case from the chamber and reloads the chamber with a live round of ammunition from the magazine. The process is repeated with each pull of the trigger until the ammunition is expended.

Rifles have a longer barrel than revolvers or pistols, and are usually more powerful and designed to shoot at targets at longer distances. Like pistols, they can be either the single shot type or the self-loading type. Rifles use various methods for the ejection of spent cartridge cases including lever, bolt, or pump action in manual operation, or recoil energy or gas pressure in automatic operation.

Shotguns can be either single or double barreled. The most common type is the design in which the barrel breaks forward on a hinge to expose the breech, into which live cartridges are manually inserted and from which spent cartridge cases are manually extracted. Some shotguns use pump action, recoil energy, or gas pressure reloading mechanisms (usually combat-type shotguns).

The barrel of a firearm is a tube made of iron or steel. The inner surface of the barrels of revolvers, pistols, and rifles contain a number of spiral grooves known as rifling. Hence they are known as rifled bore weapons. The rifling of the barrel grips the bullet and causes it to rotate, thereby preventing it from wobbling or turning over in flight.

The raised area between two grooves is called a land and the caliber of a firearm is based on the diameter of the bore (barrel) measured between two opposite lands.2 This oversimplified definition of caliber gives a rough approximation of bullet diameter as the bullet is usually slightly bigger than the diameter of the bore. Caliber is usually given in inches or millimeters and common calibers for handguns are .22" (6 mm), .25" (6.35 mm), .32" (7.65 mm), .387.357" (9 mm), .45", .455", and for rifles are .22" (6 mm), .223", and .30" (7.62 mm). There are many other calibers in existence. In fact, the suitability of a round of ammunition for use with a particular firearm depends not only on the diameter of the bullet, but also on the length and design of the cartridge case. Caliber is often a misunderstood and confusing term as the following few examples illustrate: (a) .22 L and .22 LR—the same cartridge case is used in both, but the bullet weights are different. The bore diameter is 0.215 inches. (b) .308 Winchester—it was originally designed as a sporting cartridge in the United States but is also known as 7.62 x 51 NATO, adopted by NATO as the official military cartridge. The case length is 51 mm. (c) .45 ACP and .45 Auto Rim—the same bullet is used in each of these, but the design of the cartridge case differs. The .45 ACP has no rim and is designed for use in auto pistols, whereas the .45 Auto Rim has a rim to enable it to be used in a revolver. They are not interchangeable. (d) .380 Rev, .38 S&W, .38 Special, and .357 Magnum—in each of these the diameter of the bore is approximately 0.35 inches, but the cartridge case dimensions, bullet weights, and propellant charges are different, and each is designed for a different firearm, although some interchange is possible.

With very few exceptions the inner surface of a shotgun barrel is smooth; hence shotguns are called smooth bore weapons. The caliber of a shotgun is usually expressed as its bore or gauge; the most common are 12, 16, and 20 bore with the 12 bore by far the most popular. Bore refers to the number of lead balls of bore diameter that weigh l lb.3 Smaller-diameter shotguns are usually described by the internal diameter of the barrel, for example, .410".

Overshot wad

Overshot wad

Metallic base

Filler wad

Primer mixture

Base wad centerfire round A typical shotgun round

Figure 1.1 Typical ammunition types.

centerfire round A typical shotgun

^ Shot charge

Metallic base

Filler wad

Primer cup

Primer mixture

Base wad round

Figure 1.1 Typical ammunition types.

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