Ammunition of Rifled Firearms
The unit ammunition of a small firearm is in the form of a cartridge. Typically a cartridge consists of:
- A cartridge case
- A primer
- Propellant (gun-powder)
- Bullet or projectiles
- Wads if any
Cartridge cases are usually made of brass (70% Copper and 30% Zinc). Less commonly they are made of steel. Brass, plastic and paper are used for shot shell tubes.
The main function of the cartridge case is to expand and seal the chamber against escape of gases when cartridge is fired.
There are three general shapes for cartridge cases: straight, bottle neck and tapered.
Almost all pistol cartridges are straight, whereas almost all rifle cartridges are bottled neck. The bottle neck design permits more power to be packed in a shorter, fatter cartridge.
Rifled firearms can be broadly classified into:
It is the longitudinal cutting of inside of the bore of a weapon, with a number of spiral grooves that run parallel to each other but are twisted spirally from chamber end to muzzle end. The raised portions on the bore are called lands and the lowered ones the grooves. Rifling varies in number, direction, depth and width, and these variations are reflected on the fired bullets.
Class Characteristics of Fired Bullets
When a bullet is fired down a rifled barrel, the rifling imparts a number of markings to the bullet. These markings may indicate the make and model of the gun from which the bullet was fired. They result from the specifications of the rifling, as laid down by the individual manufacturers.
These characteristics are:
- Number of lands and grooves
- Diameter of lands and grooves
- Width of lands and grooves
- Depth of grooves
- Direction of rifling twist
- Degree of twist
Individual Characteristics of Fired Bullets
In addition to class characteristics, imperfections on the surfaces of the lands and grooves score the bullets, producing individual characteristics. These individual characteristics are peculiar to the particular firearms that fired the bullets. They are as individual as finger prints.
Bullets fired from a gun will have striae (linear grooves) imparted as a consequence of transversing the barrel and may help in identification of the weapon.
Mechanism of Bullet and Shots Injury
- Basic principle is kinetic energy = ½ mv2
- For damage to occur, some or all of the kinetic energy of the missile has to be absorbed by the target tissues, where it is dissipated as:
- Mechanical disruption
To ensure transfer of energy to the tissues, some missiles are especially designed or modified to slow up or stop within the body, e.g.
- Soft headed bullets
- Air cavity within tip
- Explosive tipped bullets
- Bullets to be fired in confined places e.g. in aircrafts, to combat hijacking
Orientation (or impact) of Bullet
A bullet striking sideways will produce more damage due to more contact between the projectile and the tissues, thus transferring more kinetic energy.
Speed of Bullet
Projectile up to speed of sound i.e. 100 ft/sec inflicts damage mainly due to mechanical disruption of the tissues (e.g. revolver).
Supersonic bullets i.e. those with a speed above the speed of sound in air have additional damaging phenomena:
- Shock wave of compression
- Cavitation phenomenon –due to acceleration of tissue molecules around the track of bullet
- ‘Suction’ by near vacuum
- Ricochet phenomenon within body
- Secondary missiles within body
Factors Affecting Missile Wounds
Nature of firearm wound depends upon the following factors:
- Weapon –smooth bored or rifled
- Muzzle velocity
- Nature of projectile
- Nature of propellant
- Range of discharge
- Angle of discharge
- Degree of choke/if any
Terminal (Wound) Ballistics
- The bullet or shot initially stretches the skin and then penetrates into it
- Therefore there is an abraded or contusion collar around the entry wound
- Due to transfer of metallic particles due to rub of the bullet, there will be a smudge, or dirt ring around the wound. It may contain metal as well as grease elements of the cartridge.
- In close distance firing, there may be blackening of clothing or skin around the entry wound due to deposition of carbon particles on the skin surface (can be washed away)
- Tattooing of skin around entry wound may be produced when carbon particles are deeply thrust into the skin (cannot be washed away)
- In close wounds, due to heat effect of hot gases, there may be scorching of tissues around the wound.
Rifled Firearm Injuries
The injuries may be classified into:
- Contact wounds
- Close range wounds
- Medium/distant range wounds
- Exit wounds
- Wound is circular and smaller (may be stellate shaped over scalp)
- Burning/blackening of immediate wound edges
- Soot within the tissues may be found (usually is less)
- Muzzle mark/imprint
- Inverted margins of the wound
- ‘Back spatter’ i.e. blood is sucked up into barrel of the weapon (not common)
Close Range Wounds
- Almost always circular in shape
- Edges are inverted
- Collar of abrasion
- ‘Metal fouling’
- Burning effects (within a few centimeters)
- Tattooing around (or stippling)
Medium/Distant Range Wounds
- From less than 0.5m to several kilometres entry wounds appear alike
- There is abrasion collar around entry wound
- Wound is inverted
- Ring of dirt or grease ring
- A bullet striking sideways may produce a rectangular lacerated wound
- Exit wound depends upon:
- Intactness of bullet
- Secondary missiles generated within the body
- If bullet is not seriously deformed in the body, the exit wound is classically of a stellate shape, with triangular skin flaps
- Entry and exit may appear similar
- There may be multiple exit wounds due to secondary missiles
- Single bullet may produce multiple entry and exit wounds (multiple entry wounds are due to separate entrance wounds of jacket and core from same bullet)
- Fragmentation of bullet and intermediate target may produce irregular entrance wound
- Shored exit wound shows abrasion around exit wound, when a person standing against a wall or lying on the ground is fired.
Shot Gun Injuries
Breach end is the rear end, while muzzle end is the front end of the barrel.
In smooth bored guns, the muzzle end may be slightly constricted to control the degree of dispersion of pellets.
Full choke means the constriction of the order of forty thousandths of an inch. Interpretation of distance, which is based on dispersion of pellets, may be wrong due choke.
Device used to adjust the choke.
The number of pellets that can be made from 1 lb of Pb, which have diameter equal to the barrel.
Cartridge is a completely assembled round of ammunition, which includes:
- Cartridge case
- Powder charge
- Wadding and pellets
Slug is a small solid projectile for a smooth bore weapon, generally pellets are the projectiles in shot guns.
Following constituents of the cartridge may contribute to the wound:
- Lead pellets
- Soot (smoke and debris)
- Un burnt and burning propellant particles
- Flame/hot gases under pressure
- Detonator constituents
- Carbon monoxide
The appearance of shot gun wound is greatly modified by variations in the range of fire into:
- Contact wound
- Near contact wound
- Short range wound
- Distant range wound
Shotgun Contact Wound
When muzzle is placed firmly against body surface, contact wound is found.
- A single wound of entry is present
- Generally circular in shape
- Size is equal to bore of the weapon
- Split off skin in scalp may cause a cruciate, stellate or ragged shaped wound
- Tattooing, blackening and burning is minimal as the skin acts as a seal against the muzzle
- Wads inside wound may be found if muzzle is not pressed firmly
- Muzzle impression
- Rarely a ring mark adjacent to the wound due to contact of double barrel
- Pinkish discoloration of tissues in the wound track due to carbon monoxide.
Shotgun Near Contact Wound (or close range wound approx. 6 inches)
- Clothing will trap most if not all of the soot and powder grains of near contact fire
- On naked skin following features are produced:
- Singeing of hair, ‘clubbed hair’
- Skin burns due to flame/hot gases
- Smoke/soot soiling around the wound
- Tattooing around the wound
- Partly burnt/unburnt flakes or grains of propellant on the surrounding skin
- Wound is circular or elliptical, depending upon the angle of fire
- Edges are nibbled or irregular due to individual pellets
- Distant bruising around wound due to gas pressure trauma
Short Range Wound (approx. 6 inches to 6 feet)
- Soot soiling vanishes after 1 foot
- Tattooing may be seen
- Wound is single but with crenated margins
- Rat-hole injury
- Separate wad injury as an abrasion or bruise
Distant Rage Wound (>6 feet)
- Not soot or burning effect
- Number of separate pellet holes increase progressively around the main wound
- The wad often takes the lower trajectory and may strike below the main wound (up to 15 feet distance)
- Beyond 6-10 metres, the central hole may shrink to nothing
Exit Wound in Shot Gun Injuries
- Exit wounds are uncommon on the trunk of adults
- Extremely large exit wounds may be produced in head, neck or limbs and in children
- No cavitation effect is seen
Direction of Shot Gun Wounds
- If fired at right angle to the body shape is regular and circular
- If fired at all other angles, shape is elliptical, wound edges may be shelved
- Track of wound in tissues may indicate the direction