Wednesday, August 24, 2011

Forensic Ballistic


Definitions:

  • Ballistics: Forensic ballistics is the science dealing with the investigation of the firearms, ammunitions and the problems arising from the use.

  • Firearm: A firearm is an instrument which discharges a missile by the expansive force of the gases produced by the combustion of the propellant in a closed space.



Types of Firearm:
  • Smooth bored weapon (shot gun)- eg. musket, shotgun.
  • Rifled weapon/groove bored firearms- eg. rifle, revolver, pistol, automatic pistols, machine guns.

General make up of firearm:
  • A barrel- hollow cylinder closed at the back end called breech end, open at the end called muzzle end
  • Inside of barrel consists 3 parts- 1. chamber at the breech end for the cartridge, 2. the taper in rifled arm and chamber cone in smooth bore, connects the chamber to the bore 3. the bore which lies between the taper and the muzzle
  • A bolt or block can be opened to insert a fresh cartridge into the chamber

Smooth bored firearm:
  • Bore is perfectly smooth from end to end
  • They fire round pellets
  • The hitting range is quite small
  • Muskets- smooth, bored guns used by constables, has a long fore stuck and usually takes a bayonet at the muzzle (military gun)
  • Barrel- steel tube, as long as 40-48 inches and as short as 24-30 inches
  • May be full-choke (muzzle end is 40/1000inch, less in diameter than the breech end), half-choke (muzzle end is 20/1000 inch less) and unchoked (muzzle end and breech end equal in diameter)
  • The function of choking is to decrease the spread of shot.
  • May be single barreled or double barreled.
  • Are effective upto 30-35meters.

Shot gun cartridge:
  • Consists of a case of short metal cylinder which is continuous with a card board or a plastic cylinder
  • The case is rimmed to keep cartridge in correct position in chamber
  • Length of cartridge= 5-7cm
  • The cartridge is filled from base to above with percussion cap, gun powder, a thick felt wad with card board disc lying in front and behind it, the shot and finally the retaining card board disc, over which the edges of the cartridge cylinder walls are pressed.
  • The cartridges are numbered according to the size of the shot they contain

Groove bored/rifled firearms:
  • Spiral grooves (2-20, usually 6) inside the barrel to impart spin movement to bullet, to keep the bullet in straight line to target.
  • Rifles have firing range of 1000m.
  • Striking range is small in case of pistol and revolver.
  • Rifled firearms are divided into
  1. Low velocity <360 m/s
  2. Medium velocity 360-750 m/s
  3. High velocity >900 m/s
  4. The caliber or gauze of rifle is measured between lands, not grooves made by the spirals inside the rifles.



  • Rifle may be single shot, repeating, semi-automatic, automatic.
  • Revolvers are so called because the cartridge chambers revolve before each shot to bring the next cartridge opposite the barrel
  • Revolver has a cylindrical megazine at the back of barrel and can accommodate 5-6 cartridge each in a separate chamber.
  • Effective range is 100 m.muzzle velocity is 150 m/s.

Bullet (of Rifle/revolver/pistol)
  • Traditionally made of lead with antimony added for hardness, k/a round nose soft bullet.
  • Jacketed bullets are of 2 types:
  1. Full metal jacket bullet
  2. Semi jacketed bullet with nose fully or partly exposed

Air rifles and air pistols

Compressed air supplies the propulsion power instead of ignition powder
Small velocity,80 to 100 metre per second
Striking range about 40 metres
Causes only minor injuries


Muzzle velocity:

Shot gun: 800-1000 ft/sec (240-300 m/s)
Rifles: 2000-3000 ft/sec
Pistol: 1200 ft/sec
Revolver: 600 ft/sec (150-180 m/s)
Air rifle: 400-700 ft/sec
Musket: 1150-1950 ft/sec (350-600 m/s)

Types of projectiles/bullet
  • Lead shot: Present in shot gun cartridge, are of different sizes
  • Jacketed conical bullets: fully or partially encased with copper or copper nickel, used in rifles.
  • Hardened lead bullets: in pistol and revolver
  • Tracer bullet: barium peroxide and magnesium are enclosed in base, give light as they pass.
  • Incendiary bullet: contains phosphorus, burn the tissue.
  • Tandem bullet: two bullets together entering into single entrance wound, when an old rusty weapon’s first bullet comes out only after the second fire.
  • Dumdum bullet: expands or mushrooms on striking and produces a large hole
  • Plastic bullet: baton round, used for riot control, effective upto 50-70 m, not fired to a person under 20 meter range.

Gun powder: 3 types
Black powder

Contains carbon 15%, sulphur 10%, potassium nitrate 75%
Only 44% of black powder is burnt into gas
The powder grains are black, coarse or fine, burns with production of much heat, flame and smoke
Fine grains travel 60-90 cm or more.

Smokeless powder

Contains nitrocellulose alone or with nitro glycerin
100% smokeless powder is burnt
Produces much less smoke and are more completely burnt than black powder

Semi smokeless powder

Mix of 85% black and 15% smokeless powder

Following things come out of a gun when fired
  1. Projectile
  2. Blast of highly compressed gases
  3. Particle of unburnt /partly burnt powder
  4. Smoke
  5. Flame
  6. Wads (in shotguns)
  7. Fine lead fragments (in lead bullets )
  8. Grease from the weapon
  9. Empty cartridge (smtm in shotgun
Wounds of entrance
  • In distant shots ,it s usually smaller than the projectile d/t the elasticity of skin
  • Rounded if bullet hits the body at right angles ,oval if obliquely and gutter shaped if tangentially
  • Entrance wound is smaller than the exit
  • Edges are inverted (except in fat and decomposed)
  • Characteristic of wound of entrance found at all ranges is ring of greese and gun powder surrounded by collar of abrasion or contusion
  • It may contain wads

Wounds of exit

  • Bigger than entrance wounds
  • Very irregular and torn, stellate
  • Edges everted
  • No collar of contusion or abrasion
  • No tattooing or scorching or blackening

If a bullet hits bone,
  • a clean purched out hole seen in spongy bone,
  • It is shattered to small bits d/t great velocity, spin and tailing in compact bone
  • Rebound or deviate after striking a hard bone(wound of ricochette)


Typical stellate exit wound with everted margins.
Shot gun wounds in detail based on distance:

contact wound
  • results in a circular entrance wound that is about the size of the muzzle.
  • The wound edge will be regular and often has a clean-cut appearance with no individual pellet marks apparent.
  • There will commonly be smoke soiling of at least some of the margin of the wound.
  • There may be a narrow, circular rim of abrasion around some or all of the entrance wound, caused when the gases of the discharge enter through the wound and balloon the tissues upwards so that the skin is pressed against the muzzle.

  • If the discharge was over an area supported by bone, the gases cannot disperse as quickly as they would in soft areas such as the abdomen, and the greater ballooning of the skin results in splits of the skin, which often have a radial pattern.
  • The wads or plastic cup will usually be recovered along the wound track. The tissues along the wound track will be blackened and the surrounding tissues are said to be pinker than normal as a result of the carbon monoxide contained within the discharge gases.

close discharge,
within a few cm of the surface, will also produce a wound with a similar appearance, but as there is now room for gas escape, there will be no muzzle mark.

More smoke soiling of wound can occur, and burning of skin with singeing and clubbing of melted hairs can be seen around the wound.
There is also, very commonly, powder ‘tattooing’ of the skin around the entry wound. This tattooing is due to burnt and burning flakes of propellant causing tiny burns on the skin and cannot be washed off.
Wads will commonly be found in the wound.

intermediate ranges,
between 20 cm and 1m, there will be diminishing smoke soiling and burning of the skin, but powder tattooing may persist.
The spread of shot will begin, first causing an irregular rim to the wound. Often called a ‘rat-hole’ in the USA because of the nibbled edges, the same appearance is called ‘scalloping’ in the UK.
Separate injuries caused by the wads or plastic shot containers may be seen.
At a range of over 1 m, smoke damage and tattooing do not occur.
With a normal shotgun, satellite pellet holes begin to be seen around the main central wound at a range of about 2–3 m.
.

  • important to measure the spread of the shot to establish the range at which a particular spread of shot will occur.
  • Estimates based on generalizations about the ratio of the diameter of this spread to the range should no longer be used.

long ranges,
as 20–50 m, there is uniform peppering of shot,rarely fatal.

  • Shotguns rarely produce an exit wound.
  • Exit wounds seen when fired into the head, neck or mouth. The exit wound in these cases may be a huge ragged aperture, especially in the head, where the skull may virtually explode.
  • The spread of shot will begin, Often called a ‘rat-hole’ because of the nibbled edges, the same appearance is also called ‘scalloping’ .
  • Separate injuries m/b caused by the wads or plastic shot
  • satellite pellet holes begin to be seen around the main central wound at a range of about 2–3 m.



Close discharge of a 12-bore shotgun to the chest.
There is a large hole, partly due to disruption by gas, and no
evidence of pellet scatter is seen.

spread of the shot can be seen.

Wounds from rifled weapons in details based on distance

Bullets fired from rifled weapons will commonly cause both an entry and an exit wound.
However, exceptions to this rule are numerous and many bullets are retained within the body.

Entrance wounds in detail

Contact wounds from a rifled weapon are generally circular, unless over a bony area such as the head, where splitting caused by the propellant gas is common.
  • There may be a muzzle mark if the gun is pressed hard against the skin.
  • There may be slight escape of smoke with some local burning of skin and hair if the gun is not pressed tightly.
  • Bruising around the entry wound is not uncommon.
close range up to about 20 cm, there will be some smoke soiling and powder burns, and skin and hair may be burnt, although this is very variable and depends upon both the gun and the ammunition used.

  • The shape of the entry wound gives a guide to the angle that the gun made with that area of skin: a circular hole indicates that the discharge was at right angles to the skin, whereas an oval hole, perhaps with visible undercutting, indicates a more acute angle.
  • Examination of the entry wound will show that the skin is inverted; the defect is commonly slightly smaller than the diameter of the missile due to the elasticity of the skin.
  • Very commonly, there is an ‘abrasion collar’ or ‘abrasion rim’ around the hole, which is caused by the friction, heating and dirt effect of the missile when it indents the skin during penetration.
  • Bruising may or may not be associated with the wound.

Over 1 m or so, there can be no smoke soiling, burning or powder tattooing.

At longer ranges (which may be up to several kilometres with a high-powered rifle), the entrance hole will have the same features of a round or oval defect with an abrasion collar.


Suicidal pistol contact entry wound in the temple.
The skin is burnt and split due to the gases of the discharge.


Pistol entry wound through the eyebrow with adjacent
powder tattooing. The bruising of the eyelid is due to the fractured
skull.

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