8/7/14

1

Firearms Terminology

•  Firearms Identification (also called Firearms Examination) is the process by which fired projectiles and casings are compared to determine which gun fired has fired them

•  Ballistics is the science that studies the behavior of projectiles in flight

Types of Guns

•  Long Guns – Rifles – Shotguns

•  Hand Guns – Revolvers – Semi-automatic pistols

Long Guns •  Rifles

– Many varieties •  Hunting, sniper, AK47, etc.

– Fire bullets – Extremely powerful – Ammunition can penetrate body armor, walls, doors,

vehicles •  Shotguns

– Fire “shot” (tightly packed bb’s) – Shot spreads out in cone shape – Greatest damage occurs closer to gun

Long Guns

AK 47

Modern Hunting Rifle

Hand Guns

•  Revolvers – 5 or 6 shot

cylinders – Casings remain in

cylinder

8/7/14

2

Hand Guns

• Semi-automatic pistols – Ammunition in magazines – Casings are ejected from gun

Controlled Explosion

Barrels

•  Rifled Barrel – Rifling is a series of grooves cut into the inside

of the barrel –  These grooves impart a spin on the bullet

which stabilizes it during flight and makes it go straight (like throwing a spiral)

•  Smooth-bore Barrel – No grooves are cut into the barrel

• Shotguns

Barrels

•  Barrel – A) Land – B)Groove

•  Bullet – A) Land Impression – B) Groove Impression

A B

A

B B

8/7/14

3

Bullet Class Characteristics

•  Class characteristics of bullets differ due to differences in manufacturing – Caliber – Number of lands and grooves – Width of lands and grooves – Distance between lands and grooves – Depth of lands and grooves – Pitch (the angle at which the rifling is cut) – Twist (the distance the rifling needs to spiral down the

barrel & complete a single revolution)

A Different Pitch

Bullet Individual Characteristics

•  A cross-section of a gun barrel would reveal the existence of fine grooves, or striations, running the length of the lands and grooves.

•  These striae are created by irregularities in the rifling tool.

•  They are imparted onto the bullet as it passes through the barrel.

•  No two rifled barrels, even those manufactured in succession, will have identical striation markings.

Bullet Individual Characteristics

•  Striations impressed by the rifling in the barrel •  Striations due to wear and use of gun •  Striations due to neglect and abuse of gun

Bullet Comparisons

•  These striae are used to link fired bullets to the same weapon.

Cartridge Casings

•  Brass (30% Zinc & 70% Copper) •  Steel • Aluminum •  Plastic

8/7/14

4

Cartridge Case (Casings) Class Characteristics

•  Trade Marks – Manufacturer’s name – Headstamp

•  Shape – Rimmed – Rimless

•  Caliber – The diameter of the bore (or interior barrel) of the gun,

expressed in 100ths of an inch or mm •  Composition of metals

Cartridge Case (Casings) Individual Characteristics

•  Firing Pin Impressions •  Breech Face Marks •  Ejector Marks •  Extractor Marks

Firing Pin Impressions

•  The shape of the firing pin is impressed into the soft metal of the primer cup.

•  This reveals minute distortions of the firing pin

Firing Pin Comparisons

Breechblock

•  The breechblock has striations from the machining process that individualize its surface

Breech Face Marks

•  The cartridge head is seated against the breechblock of the weapon.

•  When the bullet is fired, the cartridge case is shoved back into the breechblock with a great amount of force. This imprints the striations of the breechblock onto the cartridge casing.

8/7/14

5

Breech Face Comparisons Ejector Marks

•  Imparted to the cartridge case when it is ejected out of the weapon.

Extractor Marks

•  Imparted onto the cartridge case when the case is extracted from the barrel.

Evidence Collection

•  Photograph location and condition of firearm •  Take complete notes on condition of weapon

– Safety (On/Off)? – Hammer Cocked? – Slide Back? – Live/Fired Rounds (number and placement)? – Make/Model/Serial Number/Caliber?

Evidence Collection

•  Wear latex gloves. •  Handle all weapons by the trigger guard or textured grip

so as not to disturb possible fingerprints. •  DO NOT PUT ANYTHING DOWN THE BARREL OF

THE GUN! •  Make note of any adhering trace evidence (blood, hair,

paint, glass, etc.) •  Do not clean the weapon off. •  Unload and immobilize the weapon in an evidence box.

Evidence Collection

•  The only reason to make a mark directly on a firearm is to note the top cylinder position of a revolver

• Mark with an arrow or #1 using a sharpie • Also diagram in report and photograph

before unloading

8/7/14

6

Evidence Collection

•  Casings must be searched for diligently at a crime scene

•  They should be packaged separately in small envelopes

• Most semi-autos eject casings to the right •  If you find them, and if you know the

approximate or exact location of the victim when shot, you can determine where the shooter was

Evidence Collection

•  Bullets must also be searched for diligently at a crime scene

•  They may be embedded in walls, cars, furniture or even in the ground, and will require an intensive effort to locate

•  May be damaged (mushroomed) due to impact •  They should be packaged separately in small envelopes •  They may also be collected at the hospital from the

victim (during surgery), or by the medical examiner at autopsy if the victim dies

Mushroomed Bullets/Slugs)

Cartridge, Cased Bullet, Mushroomed Bullet, Jacketing

Gunshot Residues (GSR)

•  Not all powder is consumed when a gun is fired •  Burning and unburned particles leave the gun, propelled

forward toward target in a cloud •  Some blow back and land on suspect’s hand, arm, face,

clothes •  Some will land on target if close enough •  The distribution of GSR around the bullet hole permits

an assessment of the distance from which gun was fired •  GSR is composed primarily of lead (Pb), antimony (Sb)

and Barium (Ba)

8/7/14

7

Gunshot Residues (GSR) GSR

GSR

•  Accuracy of distance determination varies according to circumstances of the case

•  Must have actual weapon & ammunition used in order to test, compare & make accurate assessment

•  Factors effecting deposition of GSR on target: barrel length, caliber, type of ammo, type & condition of weapon

•  Distance determinations important in cases of suicide, self defense

GSR

•  On a suspected shooter… – Collected with a GSR kit – Two cylinders with adhesive bottoms – Dab the web and back of each hand – Detected through Neutron Activation Analysis, Atomic

Absorption Spectrophotometry, or SEM – Greiss Test (photographic gel plate that reacts with nitrites) – Clothes may also be collected for microscopic exam – Face may also be dabbed (rarely done)

GSR

•  Factors affecting the presence or absence of GSR on a shooter –  Time since weapon was fired –  Behavior since weapon was fired (showering, washing hands, putting

hands in pocket, changing clothes, squirming around in the back seat of a police car, eating ,drinking, etc.)

–  Wearing gloves

•  Legitimate reasons why a person (not the shooter) might have GSR particles on him? –  Has recently (and presumably legally) fired a gun –  Works with guns or metals commonly found in GSR

Review Questions

•  What are the individual characteristics of a fired bullet? •  What are the individual characteristics of a fired

casing? •  What is the difference between Firearms Identification

and Ballistics? •  What is a false positive result for GSR? What might

cause it? •  What is a false negative for GSR? What might cause it?