Ballistics

The vast majority of

U.S. homicides involve

guns. And they are more

powerful than ever.

Lansing State Journal, July 2007

1998--Deaths due to handgunsUS 11,789Germany 373Canada 151Australia 57England and Wales 54Japan 19

In 2004 there were 29,569 gun deaths56 percent suicides40 homicide

Introduction

• Ballistics- is the study of bullets and firearms– Firearm = a weapon, such as a gun, capable of firing a

projectile using a confined explosive• Questions that can be answered by analysis of ballistic

evidence– What type of firearm was used?– What was the caliber of the bullet?– How many bullets were fired?– Where was the shooter standing?– What was the angle of impact?– Has this firearm been used in a previous crime?

History of Gun Powder and Firearms• Gunpowder invented over 1,000 yrs ago by chinese

– Gunpowder is a mixture of potassium nitrate (saltpeter), charcoal, an sulfur

– When ignited gunpowder expands to six times its original size causing a violent explosion

• Matchlock –used wicks to ignite the gunpowder• Flintlock

– Muzzle (barrel) Loaders• Gunpowder and projectile packed down barrel

– Barrel = the long, metal tube that

guides a projectile out of the firearm

– Muzzle = end of the barrel of a

firearm

• Percussion• Cartridge and bullet

Long Guns and Handguns• Long Guns

– Rifles- fire bullets– Shotguns- fire either small round pellets (shot) or a single projectile called a slug

• gauge = the number of lead balls matching the barrels diameter that it takes to weigh one pound

• Handguns– Pistols- handguns that can be fired with one hand

• Single shot initially• not as powerful and/or accurate as rifles• Today, can be classified as revolvers or semiautomatics

– Revolvers- have a cylinder which contains several cartridges that can be fired in rapid succession» Invented by Samuel Colt, patented in 1835» Cylinder can hold several cartridges

– Semiautomatic» Hold 10 cartridges in one magazine (clip)

• Fire one bullet per pull of trigger• The empty cartridge ejects and the next cartridge advances automatically

– Fully automatic» Fire repeatedly as long as trigger is pressed

Firearms and Rifling• An archer will hit a target with greater

accuracy if there is a twist on the end of the arrow feathers

• Same applies to guns and bullets– Barrel has lands and grooves– Rifling = spiral pattern of lands and grooves in

the barrel of a firearm• Lands = raised area• Grooves = indentations

– Lands and grooves cause bullet to spiral like a football

– The “rifling pattern” left on the bullet is specific to each firearm

• No two guns, even of same model, are rifled the same when made

• Provides individual evidence

Anatomy of a Cartridge

– Cartridge = a case that holds a bullet, primer powder, and gunpowder

– Shell Casing = the metal (usually brass) housing from the gunpower of a firearm

– The bullet can be composed of lead copper, or a combination of various metals

• Lead• Lead alloy (lead mixed w/ other metals• Semijacketed- thin layer of brass coating• Fully jacketed-jacketed-completely covered with brass

– Greater penetration power

– The primer powder mixture initiates the contained explosion that pushes the bullet down the barrel

• Struck by firing pin of firearm, pressure caused ignition

– The anvil and flash hole provide the mechanism of delivering the explosive charge from the primer powder to the gunpowder

– The headstamp on the bottom of the cartridge case identifies the caliber and manufacturer

How a Firearm Works• Pull the trigger and the firing pin of the

firearm hits the base of the cartridge, igniting the primer powder mixture

• The tiny explosion (not much more than a spark) of the primer powder misture on the anvil delivers a spark through the flash hole to the main gunpowder suppy

• The main gunpowder supply ignites, and the pressure of the explosion pushes the bullet from the casing and into the barrel of the firearm. The amount of gunpowder and mass of bullet determines the speed of the bullet

• The bullet follows the lands and grooves pattern of the barrel and begins to spiral before it leaves the barrel

Caliber of the Cartridge• Caliber- a measure of the inside diameter of a

firearm barrel• Bullets (and their cartridges) are name by caliber

and length. • The caliber is a measure of the diameter of the

cartridge.22, .25, .357, .44, and .45 are common calibers– Example .45 = 45/100 of an inch

• European method using metric system (example 9mm)

– Bullet removed from wound or crime scene can be linked to a gun by its caliber size

Marking on Bullets

• Lands and grooves rifling pattern in barrel of gun leaves a unique pattern on bullet– suspected weapons can be test fired and bullets

compared for individualizing evidence

Marks on a Spent Cartridge

• Marks left on bottom of cartridge by firing pins, can be analyzed• Breechblock marks are produced on cartridge as it travels in

opposite direction of bullet– Breech is the rear part of a firearm barrel

• Where cartridge is loaded, opposite muzzle

– Cartridge hits breech block and mark is left– suspected weapons can be test fired and cartridges compared for

individualizing evidence• Some other marks

– Extractor and Ejector marks• Extractor – mechanism that places cartridge into chamber of firearm• Ejector – mechanism that removes cartridge from chamber after firearm has

fired

Gunshot Residues

• All firearms explode gunpowder and produce gunshot residue (GSR)– Residues are traces of smoke and particles of

unburned powder carried sideways from the firearm by the expansion of gases as the bullet is fired

– Gunshot residues contain nitrates and metals– Residues stick to the person holding the firearm and

leave evidence on the shooter– The amount of GSR on the victim decreases as the

distance between shooter and victim increases

Trajectory

• Trajectory is the path of the for the propelled bullet– Used to determine the position of a shooter

• Trajectory can be calculated by finding two or more reference• If ignoring gravity, it can be assumed that projectiles such as

bullets travel in straight lines• Reference points can be

– bullet holes in objects or victims– An entry point and exit point on a victim – Gunshot residue or spent cartridge casings

• Lasers can trace a straight-line path to determine the position of the shooter

Gravity and Trajectory

• Two major forces acting on a bullet after it has been fired– Forward force of the gunshot and– Downward force of gravity

• If shot from close to medium distances gravity can be negated in calculation of trajectory

• Gravity needs to be accounted for if shots are fired from greater distances

Determining Location of Shooter

Building is 60 feet away along the horizon lineBullet hole is 4 feet above the ground

Where is the shooter located?

Triangulation

• B is where the shooter is located; find the length of BC• The Abc triangle has the same proportions as the ABC triangle

• So or

• AB = 732.3”

23.9"

23.5"AB

720"AC

AB

Ac

Ab

Triangulation

• Using Pythagorean’s theorem AB2 = AC2 + BC2

• 732.32 = 7202 + BC2

• BC2 = 732.32 – 7202

• BC2 = 536117 – 518400• BC = √17717 (square root)

• BC = 133.1 inches• BC = 11.1 feet

We know that the bullet hole in the seat is four feet above the ground, so the shooter is 15.1 feet above the ground

Sample 1

• Ab = 32”• Ac = 24 “

• CALCULATE BC

Building is 40 feet away along the horizon lineBullet hole is 4 feet above the ground

Sample 2

• Ab = 22”• Ac = 14“

• Calculate BC

Building is 20 feet away along the horizon lineBullet hole is 4 feet above the ground

Bullet WoundsWhy do entrance wounds tend to be smaller than exit wounds?

Bullet enters body smallMay pick bone and tissue as it travels through bodytumbling effect

If the bullet penetrates clothing, what can fibers embedded in the wound indicate?

entrance vs exit woundWhere is gunshot residue usually found?

usually only around entrance woundsIf the gun is fired with the muzzle touching the victim’s skin, what telltale mark may show up?

burn mark on skinWill larger or will smaller caliber bullets tend to lodge within the body rather than passing through? Why?

smaller caliber-less velocity (speed)