What is Forensic Ballistics?

Forensic Ballistics is the branch of science dealing with the investigation of the firearms ammunition and the problems arising from their use.

A firearm is any weapon which discharges a missile by the expansive force of the gases produced by the burning of an explosive.

There are three main subtopics under Forensic Ballistics:

·         Internal Ballistics- refers to the activity or mechanism occurring inside the firearm.

·         External Ballistics- refers to the study of the motion of projectile after it leaves the gun barrel till the time it hits the target.

·         Terminal/Wound Ballistics- refers to the impact of the firearm once they hit the target (Terminal Ballistics) or living tissues (Wound Ballistics).

Terminal Ballistics:

Terminal ballistics is a sub-field of ballistics which deals with the study of the behaviour and effect of a projectile when it hits the target and transfers the energy to the target.. it can be subdivided into penetration potential, which is the capacity of a missile to penetrate various materials and Wound Ballistics, is the effect of projectile on the living tissue.

The minimum velocity to cause an impact, is known as the Threshold Velocity, required to penetrate the skin is 40-50m/s and for bone penetration it is 60m/s.

When a projectile strikes a human body, it depresses and compresses the skin, flesh and bone tissues underneath. The continued pressure stretches them beyond the elastic limits if the projectile has sufficient energy and creates a hole. The stretched skin regains its normal state after the hole is formed and the bullet is inside. The diameter of the hole on the skin, therefore appears, on the non-stretched skin somewhat smaller than the size of the projectile which created the wound, when it was stretched.

The projectile continues its onwards progress till it leaves the body through an exit hole, or, till its energy is spent beforehand, in overcoming the resistance. The projectile is found lodged at the end of the tunnel, in later cases.

Elements of Wound Ballistics:

Wound ballistics has following important elements

1. Nature of target.

2. Velocity of projectiles.

3. Constructional features of projectiles.

       4. Range.

The Target Site:

About 43% of the total body area covers body paris (organs, cavities, nerves, blood vessels, etc.) which are easily damaged.

A Likewise, the depth of tissue on the frontal side of the body is 0.6 cm only, whereas average depth us more than five times (3.3 cm) on the back side. Thus, a frontal attack is likely to cause more serious wounds than the injury from the back side.

The velocity

The energy of the projectile is proportional to square of the velocity (E=1/2mv²). If we double the velocity, the energy of the projectile becomes four times.

The wounding effect of velocity considerations can be studied into four velocity zones:

1. Velocities under 400m/s

No abnormal wounding phenomenon is observed. The projectile does not fragment or deform to any appreciable extent unless it strikes a bone.

2. Velocities from 400-600 m/s

·         lead bullets and expanding bullets start mushrooming. Extent of mushrooming depends upon the striking velocity and constructional material and features of the projectile

·         The tissue damage is greater because the cross-sectional area increases as the projectile progresses forward.

3. Velocities from 600-1100 m/s

·         The high velocity projectile creates a vacuum behind it. As it enters the body the air behind rushes at a terrific speed and disrupts the body tissues.

·         The body parts hit by the projectile themselves become effective secondary missiles and cause extensive damage

·         The projectile fragments and these fragments cause extensive damage

·         The gyratory motion is very high due to which the tissue that comes in contact with the projectile are set into motion and churned.

4. Velocities beyond 1100 m/s

The projectile compresses air in its front due to which it produces a shockwave. The intensity of shockwave depends upon the velocity of the projectile and the energy transferred by it to the target. It forms permanent cavity and temporary cavity.

A temporary cavity is formed when the surrounding tissues absorb the kinetic energy of the bullet. It is many times larger than the size of the projectile.

A permanent cavity is the actual cavity or the line followed by the bullets the trajectory or path inside the tissues.

The range

The increasing range of fire in addition to reducing the velocity of the projectile influences the wounding effect in the following ways:

1. The lighter ejecta is dispersed. It does not reach or enter the target.

2. The initial instability of the projectile is overcome.

The powder gases, smoke, semi burnt or unburnt powder residue, metallic particles, dirt and wads if any constitute lighter ejecta. They are important in the wounding effects only when the firearm is fired, pressed to the skin(contact wound) or from very close range (near contact wounds). The expansion of gases takes place inside the body.

Characteristics of firearm injuries:

ENTRY WOUND- The wounds have certain characteristics which permit their identification without difficulties, most of the times. The prominent features utilized for the purpose are

1. The wounds are circular or oval in most of the cases. Key hole wounds are also formed by wobbling bullets.

 2. The diameter of the entrance hole is, ordinarily slightly less than the diameter of the projectile creating the hole.

 3. The edges are compressed inward – they are inverted.

 4. A contusion ring is found around the wound in most of the times. The ring is dark red to bluish-black depending upon its age.

 5. The dirt or wipe ring is not always present but whenever it is present, it is a sure sign of an entry wound.

6. Burning of skin, flesh or singeing of hair is caused when the shot is fired from a close range. The scorched skin, when it is available, it identifies the entry wound.

 7. GSR deposits are from close range firing only. They also identify the entrance wound whenever they are available.

8. The presence of a muzzle impression around the wound.

9. Sometimes the bullet carries the GSR in its flight from the ejecta, from the barrel fouling and deposit on the edges or inside the entrance hole.

 Extraneous deposits around the wound are from the following sources

1. Propellant burned powder (smoke), semi-burnt and unburnt propellant.

2. Primer residue.

3. Projectile, Cartridge Case and barrel material (from fouling and bore scraping).

 4. Intermediate targets.

The extent of extraneous deposit depends upon

1. The weapon.

 2. The ammunition.

 3. The range.

 4. The angle of fire.

 5. The target characteristics.

CHARRING, SCORCHING, BURNING, SINGEING - These are the effects of flame or hot gases produced in the combustion of propellants. The charring is caused when the shot is fired from a very close range. The size, shape and extent are characteristic of the firearm and range. The Charring is often confused with the Blackening, Tattooing, Dirt Ring or even with Contusion Ring. The Charring is different from Blackening. The later can be removed with a cotton swab moistened with spirit while the former cannot be removed in this way.

 BLACKENING-The blackening is caused by the smoke deposits. The smoke particles are light. They do not travel afar. Therefore, smoke deposit i.e. blackening is limited to a short range. The colour of smoke is grey to black in black powder and light grey to dark grey in smokeless powder.

TATTOOING-The tattooing is also known as peppering or stippling. It is the deposit of unburnt or semi-burnt powder particles under the skin. Tattooing, ordinarily, cannot be removed with a swab.

DIRT RING OR PROJECTILE WIPE RING- The dirt ring is deposited by some projectile around the wound. The materials come from

1. The projectile may carry grease on them. The dirt gets collected on the grease which, in turn, gets deposited around the wound.

 2. Deposit of soot/GSR present on bullet. The projectile picks up the soot/GSR from the powder ejecta which rush past the projectiles inside or outside the barrel.

 3. Dirt due to intermediate target (clothes, mud walls etc.) or from the surface from which the projectile has ricocheted.

 4. In shot gun ammunition, the pellets and buck shots are rubbed with graphite. A small amount of graphite is carried by the projectiles which they deposit around the entry hole. The lead bullets may also blacken the edges of the entry wound.

 Foreign material

The projectile or their fragments and sometimes the wads are found inside the body, these may also indicate the nature of firearm used.

 CONTUSION- The edges of wound are contused by the impact of the projectile. The colour of contusion varies from reddish dark to bluish black. The contusions are in the form of a band around the wound and are often of uniform width. The tissues are ruptured and swollen.

EXIT WOUND-

All exit wounds, irrespective of range of firing, the following identifying features

1. They have no fixed shape or size. Usually they are larger than entry wound and are irregular.

2. The eversion of edges and the direction of pushed or pressed out flesh, indicate the exit wound.

 3. The presence of projectile, fixed in the exit wound.

 4. If the entry wound is established and a probe through this wound comes out of another wound. The later is obviously an exit wound.

Author: Ms. Jigisha Ash

Intern

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