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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
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