Chapter 15

Firearms, Toolmarks & Impressions

Firearms StatisticIn 2004 – 29,569 gun deaths in USA

Suicides – 56%

Homicides – 40%

In 1998 handguns killed:

373 people in Germany

151 people in Canada

57 people in Australia

54 people in England and Wales

19 people in Japan

11,789 people in USA

Firearms Statistics

Involved in 2/3 of all homicides in USA Handguns – used majority of the time

Types of Firearms• Handguns / pistols

• Revolver • Semiautomatic

• Rifles

• Shotguns

• Air or BB guns

Handguns/Pistol held in one hand

easy to carry and conceal

not accurate

limited to # of bullets

Types Revolver

cylinder that holds 6 bullets reload by hand does not eject spent shell

Semiautomatic shoots 1 cartridge each time trigger is pulled empty cartridges reloads automatically can hold 5 – 19 rounds in magazine/clip ** fully automatic – keeps firing when trigger is held back

Rifles long barrel

butt stock

shoot more powerful cartridges than hand guns therefore greater ranges and better accuracy

Shotguns similar to rifles – but used to shot a packet

of spherical pellets

inside of barrel – not rifled

Air guns and BB guns

shot/pellets propelled by pressurized air, spring compression or carbon dioxide cartridge

Parts of a gun Breech face

Firing pin

Ammunition 4 components

Cartridge case Primer

lead styphnate barium nitrate antimony sulfide

Propellant smokeless nitrocellulose powder

Projectile - Bullet

Bullets Made of lead – sometimes jacketed with brass, copper or

steel

Bullet size Caliber - measure of the diameter – expressed in inches or

metric Caliber also corresponds to inside diameter of firearm’s barrel

– bore .22 caliber = bullet size= 22/100 of an inch 9mm ≈ .38 caliber

Shot gun gauge – interior diameter of the barrel – determined by

the number of lead balls that fit the barrel to equal 1 pound

Types of Bullets

Full – metal jacket Jacketed hollow point Semi wad-cutter

Jacketed soft point Lead round nose Wad - cutter

More Bullets

Class Evidence Bullet

Weight

Dimensions

Shape

Type

Rifling - bore of a gun that has been machined to form a grooved spiral

lands and grooves

Individual Evidence Bullet

Unique markings and wear patterns cause striae

cannelures – securely fastens bullet to cartridge case

Class Evidence Cartridge

Dimension of shell casing

Head Stamp

Rim or centerfire

Individual Evidence Cartridge

Impression of the firing pin when it strikes the primer case

Breech marks – imparted to central area of casings at the primer when cartridge case is slammed backwards (Breech – portion of gun that contains firing mechanism)

Individual Evidence Cartridge

• Extractor & Ejector may leave identifying marks• Extractor – hooked or crescent shaped part attached to the

breech block which withdraws spent casings from chamber when the breechblock separates from barrel after firing

• Ejector – part on the firearm whose function it is to throw a spent casing from gun after firing

• Cartridges show scratches imparted by their movement in the magazine/clip• Magazine/clip – container that holds cartridges under

spring pressure to be fed into chamber

• Marks on the chamber caused by movement across chamber during loading and removing • Chamber – contains cartridge for firings

Firearm Evidence Summary

ClassBullet typeBullet caliberBullet weightLands and groovesRiflingCartridge caseHead stamp

IndividualStriaeFiring pin marksBreech marksExtractor marksEjector marksChamber marks

Bullet Identification