Author Topic: Ballistics  (Read 349 times)

Offline Md. Anikuzzaman

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« on: April 15, 2018, 05:38:56 PM »

What is Ballistics?
Ballistics is the area of Forensic Science that deals with firearms; how they are used, why they are used and why they are used frequently in the practice of murder.
What many people do not realise is that when a person is shot the wound and the condition of the victim can tell a lot about the nature of the weapon that has been used. Indeed if the weapon has been left at the scene of the crime - which sometimes happens when the perpetrator panics - the weapon itself can go a long way to providing valuable information as to the kind of person who has committed the offence.

What Ballistics Does
Ballistics involves the analysis of bullets and bullet impacts to determine information of use to a court or other part of a legal system. Forensic firearms examiners are trained to examine and analyze weapons, bullets, ballistics and how a bullet has behaved after being fired from a gun. Ballistics investigators often have to :
•   analyze firearms, ammunition, and tool mark evidence in order to establish whether a certain firearm or tool was used in the commission of a crime.
•   analyze bullets and shell casings found at a crime scene to determine what kind of weapon fired them
•   match a particular bullet or shell casing to a specific weapon or linking bullets or casing from multiple crimes scenes back to a single weapon
•   reconstruct crime scenes by estimating the distance between the gun muzzle and the person that was shot, and determining the angle or trajectory of the bullets fired

Branches of Ballistics
Ballistics, the science that deals with the motion of projectiles such as bullets, shells, rockets, and aerial bombs. It is important to police officers who investigate crimes involving shooting, to artillerymen and naval gunnery officers, and to engineers who design firearms, missiles, bombsights, and fire-control systems.
Ballistics has three branches:
•   Interior ballistics deals with the behavior of a projectile within a gun barrel.
•   Exterior ballistics is concerned with the motion of a projectile in flight.
•   Terminal ballistics deals with the effect of the projectile on its target.

Interior Ballistics
A firearm such as a rifle, pistol, or artillery piece fires a projectile as a result of the burning of its propellant, which usually is smokeless powder. When the powder is ignited, large quantities of gases are produced. These gases force the projectile through the barrel in much the same way that children blow peas from a pea-shooter. If the gas pressure is too small, the bullet or shell will not reach its target. If the pressure is too great the gun may blow up. Also, if the pressure changes with each shot, the velocity of the projectile will change and accuracy will be poor. How to regulate this pressure is one of the most important problems in interior ballistics.
Almost all types of firearms (except for shotguns and rocket launchers) have spiral grooves on the inside of their barrels. These grooves are called rifling. The depth, diameter, and number of turns vary in different weapons and differ slightly with each separate weapon of the same type. When a pistol or rifle is fired, the bullet's metal fills into the grooves. Irregularities, some visible only through a microscope, leave markings on the bullet. These markings help ballistic experts determine which weapon fired a given bullet, since all bullets from the same gun have similar markings. This type of investigation is useful in police work.

Exterior Ballistics
When a bullet or artillery shell leaves a gun, it spins like a top because of the spiral rifling. This spinning motion gives the projectile stability of flight. Some rocket missiles are made to spin by metal fins or by small auxiliary rocket engines. While in the air, the projectile is subject to various forces, such as gravity, air resistance, wind, and drift" caused by the spin. The effect of these forces must be considered when aiming a weapon to achieve accurate shooting.

Terminal Ballistics
The penetration of tank armor by armor-piercing ammunition is an important concern of terminal ballistics. Another is the damage done to the target by the blast effect and flying fragments of a shell or bomb. With nuclear weapons there is also damage from intense heat and radiation. The study of terminal ballistics helps scientists to develop more effective weapons and to devise means of defense against enemy weapons.

Types of Firearms
Firearm means any portable barrelled weapon that expels or is designed to expel or may be readily converted to expel a shot, bullet or projectile by the action of an explosive.
A firearm is a dynamic system for delivering maximum destructive energy to a target, in the form of a high-velocity bullet, with minimum delivery of energy to the shooter.
There are various types of firearms:
   Rifle
   Shotgun
   Machine gun
   Sub-machine gun
   Pistol
   Revolver

A relatively long-barreled firearm, fired from the shoulder, having a series of spiral grooves cut inside the barrel (a process called ‘rifling’) imparting a rapid spin to a single projectile.

A shoulder-fired long gun with no rifling in the barrel, designed to shoot a large number of small projectiles (“shot”) rather than a single large projectile (“a bullet”).

Machine Gun
A machine gun is a fully-automatic firearm. This means the weapon will continue to load and fire ammunition until the trigger, or other activating device, is released, the ammunition is exhausted, or the firearm is jammed.

Sub-machine Gun
A hand-held, lightweight machine gun consisting of relatively low-energy handguntype cartridges and fired from the hand, hip, or shoulder

A revolver is a firearm that has a cylinder with a number of chambers. These chambers are designed to be manually loaded with cartridges of the appropriate caliber and then, as the cylinder rotates into position under the hammer, the trigger can be pulled, releasing the hammer causing the cartridge to be fired.

Pistols are firearms designed for a more automatic operation. Cartridges are loaded into an ammunition magazine which is inserted into the firearm. As long as cartridges are present in the ammunition magazine and the firearm is functioning properly, the action of the firearm is responsible for the feeding and chambering of the cartridge and the extraction and ejection of the cartridge case once the cartridge is fired. They can be designed to fire semi-automatically or fully automatic.
Semi-automatic operation requires a pull of the trigger to fire each cartridge. Fully automatic operation allows for multiple cartridges to be fired with a single trigger pull for as long as ammunition is available to be fired.

Craft Weapons
Ccraft production of small arms refers principally to weapons and ammunition that are fabricated largely by hand in relatively small quantities. Craftproduced small arms range from rudimentary pistols and shotguns to more advanced assault rifles. The homemade guns are included in this category. Homemade guns are crude firearms roughly made from basic, household materials.

Classes of Firearms
All firearms, whether military assault rifles or civilian pistols, are classified into three broad categories:
•   fully automatic;
•   semiautomatic; and
•   other.
The groupings are based on how the weapon fires and loads bullets into its chamber for the next firing.

Automatic Weapons
Automatic weapons are manufactured for the battlefield to fire a continuous stream of bullets from attached magazines or drums as long as the gun's trigger is depressed. The escaping gas of each bullet fired is mechanically used to prepare and fire the next bullet and to eject spent shells.
These weapons are often called machine guns and have been federally banned for civilian sale and ownership, with few exceptions, since the mid-1930's.

Semiautomatic Weapons
Semiautomatic weapons fire one bullet with each pull of the trigger but can fire dozens of bullets from a magazine without reloading.
Other firearms
Other firearms, including revolvers, breach-loaded and pump-action guns, fire one bullet with each pull of the trigger. But they are not considered semiautomatic because they do not reload automatically. They use a variety of methods to prepare up to eight bullets for firing before reloading.

Modern ammunition takes the form of integrated, self-contained cartridges,
integrating three key elements in one unit:
•   a bullet, the actual projectile that is expelled from the firearm’s barrel;
•   propellant, which generates the force and pressure needed to put the bullet in motion and into flight; and
•   a primer, which in modern usage is a volatile and pressure-sensitive chemical mixture that is responsible for igniting the propellant.

The use of a chemical primer to ignite the propellant dates back to the development of the percussion cap in the early 1800s, when it was discovered that striking a cap containing fulminate of mercury created a flame that could then move into the main charge of powder.
Today, the exact chemical composition of primer mixtures can vary and remains proprietary.
Lead styphnate is the main ingredient, generally, although individual primers may also include some of the following:
   trinitrotoluene (TNT)
   lead or copper sulphocyanide, lead peroxide,
   sulfur, tetryl
   barium peroxide, and barium nitrate.
Ground glass may also be added as a “sensitizer,” to create friction when impacted by the firing pin.
A primer mixture is a high explosive; working with it and placing the primer in the case are extremely sensitive parts of the ammunition manufacture process.

Rimfire vs. Centerfire
Rimfire cartridges were first developed in the 1800s, and rimfire ammunition remains in heavy usage in .22 caliber cartridges. As the name implies, the primer composition is spun into the rim of the cartridge case, putting it in immediate contact with the powder propellant.

By comparison, centerfire ammunition has a cylindrical cap seated in the cartridge head that contains the primer mixture. The cap consists of a cup-and-anvil combination and a pellet of primer mixture. During firing, the firing pin compresses the primer composition between the cup and anvil, causing a flame that passes through a hole or vent to ignite the propellant charge. Practically, the development of the centerfire system was the great milestone in weapon and ammunition development; with it, only the primer cup needed to be soft enough to be crushed by the firing pin, freeing the main body of the cartridge case to be harder, providing a gas seal for much higher pressures than could be obtained with rimfire ammunition.

Centerfire cartridges also developed, in part, due to the desire to reuse the most expensive part of the cartridge, the case; the centerfire configuration permits new primer assemblies to be inserted into expended casings.

Firearms Identification
Firearms identification is similar to fingerprint examination only to the extent that both processes involve the comparison of known items with unknown items from the crime scene. Otherwise these are two entirely different fields of endeavor. For that reason two different  individuals generally carry out firearms examinations and fingerprint examinations.

The term firearms identification is often used in conjunction with the term tool mark identification. In reality, much of firearms identification entails a specific area of tool mark identification. By definition a tool mark results from the contact of one surface with another, the harder of which is the “tool.” Thus, in the case of a firearm and a bullet, the firearm (for instance the interior of the barrel) is the tool that produces tool marks on the surface of the bullet as it moves through the barrel upon discharge of the firearm. Likewise, the examination of firing pin impressions, magazine marks, extractor marks, ejector marks, breech face marks, and chamber marks on fired cartridge cases all constitute tool mark examinations.

The fact that firearms identification also involves examinations other than tool marks accounts for the distinction between the two areas.
The analysis of gunpowder patterns on clothing, the determination of cartridge case ejection patterns, and the measurement of trigger pull or establishing bullet trajectory are examples.
Likewise, weapons function testing, shot pellet pattern testing, and serial number restoration are additional non–tool mark comparison aspects of firearms identification.

The Beltway Sniper Case
   In 2002 a sniper murdered a series of people who were refueling their vehicles at gas stations and convenience stores in the Washington, D.C., area, gripping area residents in fear for weeks. Members of the firearms identification unit of the Bureau of Alcohol, Tobacco and Firearms (ATF) came to the scenes of the various shootings in an effort to determine where the shots were fired from and to see if any other evidence could be found at the scenes.
   The firearms analysts were able to conclude that a 223-caliber rifle was involved and that one weapon was responsible. They reached this conclusion by examining markings left on the bullets recovered from the victims. Once the weapon was recovered, it was positively identified using these same markings.
   The “Beltway Sniper” was actually two men, John Muhammad and Lee Malvo, who fired a rifle from the trunk of their car. Malvo was implicated as a shooter through a fingerprint found on the weapon and a fingerprint found on a cartridge case near one of the scenes. Finding identifiable fingerprints on weapons and fired cartridge cases is pretty uncommon, making this a stroke of good luck for investigators. Muhammad and Malvo received the first of their convictions in 2004.

   Firearms examiners use a variety of demonstrative exhibits, including photographs, diagrams, scale models, and computerized animation.
   These exhibits assist in both explaining the examinations conducted and presenting the theory of reconstruction of the shooting. Television and movies have caused jurors to come to expect a high-tech presentation and it is up to the expert to rise to the occasion.

Grooves and Rifling
Handguns and long guns, with the exception of most shotguns, have spiral grooves cut or formed into the interior surface of the barrel. These grooves, called rifling, are designed to impart a spin to the bullet as it moves down the barrel upon discharge of the weapon. This spin stabilizes the bullet in flight to improve accuracy and increase the effective range. The areas between the barrel grooves are known as “lands.” There are an equal number of lands and grooves in a gun barrel and typically ranges from two to 16.

Pathological Range of Fire
Crime scene and police agencies need to ascertain several key points during the investigation of deaths involving firearms. These points include:
•   The type of weapon involved (rifled or smooth bore)
•   The type and caliber of ammunition used
•   The number of shots fired
•   The direction/angle of the shots

The pathologist, while also actively involved in determining the above, is further expected to supply accurate information in his report pertaining to:
1. The range of fire
2. Confirmation of the direction of fire
3. Internal organ damage
4. A comment of capability of defense or response on behalf of the victim

The forensic pathologist is more or less bound to categorize the range of fire based on a set of predetermined parameters. The presence or absence of certain features of the gunshot wound either include or exclude it from one of the three categories that follow. The most workable classifications for injuries caused by rifled weapons are:
•   Contact (hard contact, contact, and near contact)
•   Intermediate
•   Distant

Contact Range
Contact range implies that the muzzle of the gun is placed against, or very near to, the skin of the victim at the instant of discharge. If the muzzle is placed sufficiently firmly against the skin surface as to impart an indentation or bruise/abrasion (muzzle imprint), then the term hard contact is used. As the muzzle effectively seals the bore of the gun against the skin, little, if any, gas escapes externally.

Hard contact range discharge

Hard contact discharge against tethered skin
A classic stellate splitting radial complex of lacerations is demonstrated.
This is due to rapid gas expansion and tearing of soft tissues away from the skull at the instant of discharge.
Once reconstructed, the faithful representation of the muzzle imprint is seen. Note blackening of the wound edges — .38 Special.

Near contact and contact range discharge
This photograph demonstrates a near contact (left) and contact (right) entry wound to the temple. The contact wound shows circumferential blackening approximating the diameter of the muzzle — no distinct muzzle imprint is seen.
The near contact entry wound shows a perfectly circular but wider sooty area, representing a distinct but short distance from muzzle to skin .38 Special revolver.

Written by:
Mr Quazi MH Supan
Associate Professor
University of Dhaka