Firearms and Ballistics

Mankind has been fascinated by the idea of launching a projectile at animals for thousands of years.Gunpowder was first used in China over 1000 years ago, but was primarily used in firecrackers and only rarely in military weapons.The use of gunpowder migrated to Europe in the 14th century. Once it was established that projectiles launched with gunpowder had a devastating effect on armored knights, its use in weaponry was vastly accelerated.

History of Firearms

Gunpowder is made from a mixture of sulfur, charcoal, and saltpeter (potassium nitrite).1 mole of solid gunpowder, when ignited, will produce 6 moles of gas—hence, a big whooooosh!Add this amount of expanding gas to a small metal tube and it will drive a projectile out at high speed in a specific direction. The firearm was born.

So, Why Gunpowder?

Modern methods of manufacturing have refined gunpowder’s original ingredients to produce the maximum amount of expansion with the least amount of residue (leftover powder.)The manufacture of modern powders is standardized enough that gunpowder residue can be analyzed by methods which identify specific components, which can be used in forensic analysis.

Modern Gunpowder

Modern firearms are manufactured in a variety of shapes and sizes to fit multiple purposes. The proliferation of guns in the U.S. has made the forensic scientist’s job very difficult. The sheer numbers of guns and the astonishing variety of makes keeps the scientists very busy.Today, virtually any military gun (including automatic weapons) can be found used by gang members, drug traffickers, paramilitary “survivalists,” or the “average” citizen.

Forensic Analysis of Firearms

Handguns were conceived as an easily-toted method for self-defense. As such, handguns are compact to make them easy to hide and carry. They are heavily controlled and regulated in most states for these reasons.Since no one is guaranteed a perfect shot, handguns must be able to provide more than one shot.The two most common handgun types are double-action revolvers and semi-automatic pistols.

Types of Firearms: Handguns

Rifles differ from handguns in the length of the barrel and the presence of a butt stock. They are harder to carry, are poorly concealable, and more loosely regulated than handguns. However, they are much more accurate and shoot more powerful cartridges than handguns.Military rifles can be semi-automatic or automatic (repeated firing with a single pull.)

Types of Firearms: Rifles

Shotguns have a similar external appearance to rifles, but differ in the lack of rifling inside the barrel, which is the basis for their legal definition.A shotgun shell may contain one large projectile (called a slug), a few pellets of large shot, or many tiny pellets. Shotguns are available in single shot (break action), double barrel, pump action, and semiautomatic.

Types of Firearms: Shotguns

Automatic pistols, like the Uzi, are used by some crime syndicates but are too expensive for the average criminal.Air guns (BB guns) shoot a projectile by using compressed air, instead of gun powder. A chamber of compressed CO2 is attached. In most states, air guns are not regulated as firearms, despite both homicides and suicides involving these machines.

Other Types of Firearms

The term ballistics refers to the science of the travel of a projectile in flight. The flight path of a bullet includes: travel down the barrel, path through the air, and path through a target.How much damage the projectile can produce is dependent upon the length of the barrel. Higher velocities can be generated with a longer chamber. Rifles cause more damage.

Ballistics

In the 18th century, gunsmiths discovered that putting spiral grooves in the bore would impart a spin to the bullet that improved accuracy markedly. Modern weapons have metal lands and grooves with a twist inside the barrel of the gun, called rifling. This rifling is slightly different for each weapon, imparting different patterns of deformation on the bullet. These patterns can be used to identify a particular weapon used in a crime.

Rifling

Ammunition consists of projectiles, such as bullets and shot, together with their fuses and primers, that can be fired from guns or otherwise propelled. When shot from a gun, the bullet cartridge either stays in the chamber or is ejected after firing.

Ammunition

Bullets are made of lead; sometimes jacketed with brass, copper, or steel.Bullet size is determined by the diameter which is called the caliber or gauge. In general, the larger the diameter, the more damage that is inflicted.

Bullets

Whenever a gun is fired, the shooter gets sprayed with an invisible blast of chemical residues that are byproducts of the incomplete combustion of gunpowder, primer, and lubricants. This is called gunshot residue.Gunshot residue can also be found on the victim’s body or clothing. Depending upon how far away the victim is from the weapon, different types of residue are seen.

Gunshot Residue

If a bullet is recovered from the scene or from the body, it may be compared to bullets obtained by test-firing the suspected weapon. This is done with similar ammunition.Bullets are marked in the 12 o’clock position so that a reference point can be compared. Test firings are then made into a tank of water. The bullets are recovered and compared for similar lands and grooves. This can be very difficult if the bullet has been deformed after firing.

Firearms Evidence

Class characteristics refers to the type of caliber and rifling. Rifling pattern may turn to the right or left, with a given rate of twist. The number and depth of grooves can vary also. The same type of gun will produce class characteristics.Individual characteristics refers to imperfections within the barrel of a particular gun. These comparisons allow a particular bullet to be linked to a specific firearm.

Two Types of Bullet Characteristics

• Bullet Type• Bullet Caliber• Bullet Weight• Lands and Grooves• Rifling• Cartridge Case• Head Stamp

Class Characteristics

• Striae• Firing Pin Marks• Breech marks • Extractor marks• Ejector Marks• Chamber Marks

Individual Characteristics

Bullet and cartridge markings cannot be used with shotguns, since they fire shot-containing cartridges. In many cases the shot rides inside a plastic sleeve that prevents it from ever touching the barrel, and even in cases where the shot does touch the barrel, the random movement of the shot down the barrel will not leave any consistent marks. However, shotgun cases can still be examined for firing pin marks.

Ballistics Limitations