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Home , Breechblock, Gun barrel

8/7/14

Firearms Terminology

• Identification (also called Firearms Examination) is the process by which fired and casings are compared to determine which fired has fired them • is the science that studies the behavior of projectiles in flight

Types of Long Guns • • Long Guns – Many varieties • Hunting, sniper, AK47, etc. – Rifles – Fire – – Extremely powerful – Ammunition can penetrate body armor, walls, doors, • Hand Guns vehicles – • Shotguns – Semi-automatic – Fire “” (tightly packed bb’s) – Shot spreads out in cone shape – Greatest damage occurs closer to gun

Long Guns Hand Guns

AK 47 • Revolvers – 5 or 6 shot cylinders – Casings remain in Modern Hunting

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Hand Guns Controlled Explosion

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

B Barrels A Barrels

• Rifled Barrel • Barrel – is a series of grooves cut into the inside – A) Land of the barrel – B)Groove – These grooves impart a spin on the • Bullet which stabilizes it during flight and makes it go – A) Land Impression straight (like throwing a spiral) – B) Groove Impression • Smooth-bore Barrel – No grooves are cut into the barrel A • Shotguns B B

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Bullet Class Characteristics A Different Pitch

• Class characteristics of bullets differ due to differences in manufacturing – – 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)

Bullet Individual Characteristics Bullet Individual Characteristics

• A cross-section of a would reveal the existence of fine grooves, or striations, running the • Striations impressed by the rifling in the barrel length of the lands and grooves. • These striae are created by irregularities in the rifling • Striations due to wear and use of gun tool. • Striations due to neglect and abuse of gun • 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 Comparisons Casings

• These striae are used to link fired bullets to the • (30% Zinc & 70% Copper) same weapon. • Steel • Aluminum • Plastic

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Cartridge Case (Casings) Cartridge Case (Casings) Class Characteristics Individual Characteristics

• Trade Marks • Impressions – Manufacturer’s name • Breech Face Marks – Headstamp • Shape • Ejector Marks – Rimmed • Marks – Rimless • Caliber – The diameter of the bore (or interior barrel) of the gun, expressed in 100ths of an inch or mm • Composition of

Firing Pin Impressions Firing Pin Comparisons

• The shape of the firing pin is impressed into the soft of the cup. • This reveals minute distortions of the firing pin

Breechblock Breech Face Marks

• The has striations from the machining • The cartridge head is seated against the process that individualize its surface 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.

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Breech Face Comparisons Ejector Marks

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

Extractor Marks Evidence Collection

• Imparted onto the • Photograph location and condition of cartridge case when • Take complete notes on condition of weapon the case is extracted from the barrel. – Safety (On/Off)? – Hammer Cocked? – Slide Back? – Live/Fired Rounds (number and placement)? – Make/Model/Serial Number/Caliber?

Evidence Collection Evidence Collection

• Wear latex gloves. • The only reason to make a mark directly on • Handle all weapons by the guard or textured grip a firearm is to note the top cylinder position so as not to disturb possible fingerprints. of a • DO NOT PUT ANYTHING DOWN THE BARREL OF • Mark with an arrow or #1 using a sharpie THE GUN! • Also diagram in report and photograph • Make note of any adhering trace evidence (blood, hair, paint, glass, etc.) before unloading • Do not clean the weapon off. • Unload and immobilize the weapon in an evidence box.

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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 Mushroomed Bullets/Slugs)

• 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

Cartridge, Cased Bullet, Mushroomed Bullet, Residues (GSR) Jacketing • 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)

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Gunshot Residues (GSR) GSR

GSR GSR

• Accuracy of distance determination varies • On a suspected shooter… according to circumstances of the case – Collected with a GSR kit • Must have actual weapon & ammunition used in – Two cylinders with adhesive bottoms order to test, compare & make accurate – Dab the web and back of each hand assessment – Detected through Neutron Activation Analysis, Atomic • Factors effecting deposition of GSR on target: Absorption Spectrophotometry, or SEM barrel length, caliber, type of ammo, type & – Greiss Test (photographic gel plate that reacts with nitrites) condition of weapon – Clothes may also be collected for microscopic exam • Distance determinations important in cases of – Face may also be dabbed (rarely done) suicide, self defense

GSR Review Questions

• Factors affecting the presence or absence of GSR on a shooter • What are the individual characteristics of a fired bullet? – Time since weapon was fired – Behavior since weapon was fired (showering, washing hands, putting • What are the individual characteristics of a fired hands in pocket, changing clothes, squirming around in the back seat of a casing? police car, eating ,drinking, etc.) – Wearing gloves • What is the difference between Firearms Identification and Ballistics? • Legitimate reasons why a person (not the shooter) might have • What is a false positive result for GSR? What might GSR particles on him? cause it? – Has recently (and presumably legally) fired a gun – Works with guns or metals commonly found in GSR • What is a false negative for GSR? What might cause it?

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