Chapter 17Firearms

Firearms (not ballistics –study of projectiles)

• http://www.youtube.com/watch?v=XIayrDbVdJ0

• http://www.youtube.com/watch?v=Cgn1nhUEgo8• Inside the Oakland Crime lab- ballistics• Britain’s CSI School- ballistics•

JFK Assasination (3 min in- also power point- lindbeck

Cold case JFK Nova

Bullet Comparison

• Firearms Identification: – people identify whether a bullet/cartridge was

fired by a weapon– Identifier has knowledge of types of weapons,

serial # restoration, gunpowder detection, distance determination

– Firearms Identification System (IBIS)– Ballistic Identification system (same)

• Grooves:– Cuts or low lying portions between lands in the

barrel of the gun

• Rifling: – Spiral grooves that are formed in the bore of the

barrel to impact a spin to the projectile

• Bore:– Interior of fire arm barrel

• Lands:– The raised portion between grooves

• Caliber:– Diameter of the bore (distance between

opposite lands) measured in 100th of an inch or mm (Typical: .22,.32,.38,.45 or 9mm)

.22 .32 .38

.45 9mm

Rifling Process

• Broach cutter (series of concentric rings, each bigger than the next)

• button process(steel plug w/ grooves forced through barrel, no cutting)

• mandrel rifling (rod of steel is inserved into oversized barrel which is compressed)

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

• Riflings (pitch and twist), lands and caliber are class characteristics while striations are individual to a barrel. These characteristics are impressed onto bullets

• Examine test and evidence bullets w/ a comparison microscope

Shotguns

• Most have smooth barrels w/ diameter in gauge (Note: can have rifled shotgun barrels)

• Higher the gauge the smaller the diameter

Note: FBI maintains General Rifling Characteristics File ( data on known weapons)

Cartridge Cases

• Breechblock: rear part of firearm barrel

• Extractor: mechanism in a firearm by which a cartridge of a fired case is withdrawn from the chamber

• Ejector: mechanism in a firearm that throws the cartridge or fired case from the firearm

• The act of pulling a trigger serves to release the weapon’s firing pin, causing it to strike the primer, which in turn ignites the powder. The expanding gases generated by the burning gunpowder propel the bullet forward through the barrel, simultaneously pushing the spent cartridge with equal force against the breechblock. The shell is impressed with markings by its contact with the metal surfaces of the weapons firing and loading mechanisms

• Firing a gun

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 the same amount of force. This imprints the striations of the breechblock onto the cartridge casing.

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.

• Cartridge case is impressed w/ markings by contact w/ the firing pins and loading mechanism (individual char.)

Breechblock Firing Pin

• Firing pin, breechblock, extractor and ejector marks may also be impressed onto shotgun shells

• Cases give distinctive points of comparison for individualization (get random striations markings, imperfections, surface markings)

Bullet Characteristics

Class• Number, width,depth

of lands and grooves (on a bullet the lands will be an indentation the grooves a raised portion)

• twist

Individual• Striae impressed by

barrel• Striae due to wear and

use• Striae due to neglect

and abuse

Cartridge Case Characteristics

Class• Trademarks

(manufacturer’s name, head stamp)

• Shape (rimmed, rimless, straight)

• Caliber• composition

Individual• Firing pin impressions• Breech face marks• Ejector marks• Extractor marks

Forensic Chemistry Lab (starts with striations, ends with serial id)

Automated Firearm Search Systems

• Allows FBI to store bullet and cartridge surface characteristics

• Allows a network of jurisdictions to share info

• DRUGFIRE: FBI system emphasizing unique markings on cases (esp. guns w/ gang or drug background)

• IBIS: Integrated Ballistic ID system – Digital microscopic images of identifying

features on expended bullets & cartridge cases– Software used: Bulletproof & Brasscatcher

Gunpowder residue

• Firing distances are approximated by the study of spread and density of gunshot residue (GSR)

• Chemicals Left Behind:– Unburned & partially burned particles of

gunpowder and smoke (get deposited on hands, clothes)

– Vaporous lead– Ammunition is a blend of lead, BaNO3,

antimony sulfide

Distances

• Contact (gun touching victim)– Heavy concentration of GSR, scorch marks,

fiber melt, blow back produces a stellate (star tear pattern)

• 12-18 inches– A halo of GSR, scattered specks of unburned

and partially burned powder

• > 3 feet– No GSR– bullet wipe-visual indication hole; dark ring of

carbon, dirt, lubricant, primer that wipes from bullet surface

Bullet wipe

• Shotgun– Measure spread of discharge shot– Approx. 1 in. per yard

Studying Gunpowder on Clothing

1. Examine microscopically

2. If lack of color contrast or blood is present – use infrared photography

3. Chemical Test (for nitrates)- Gneiss- develops pattern- Sodium Rhodizonate w/ acid- GSR

Blood GSR Blood GSR –w/ IR

Stripped Clothing

Gneiss Test

Sodium Rhodizonate

Studying Gunpowder on Hands, body

• Old Test- dermal nitrate (lack of specificity- tests (+) w/ other substances

• Now- detect barium & antimony– Measure amts. on persons hand– Swab hand w/ 5% soln of nitric acid w/in 6

hours of firing, then analyze w/:• Neutron activation analysis

• Flameless atomic absorption

• Scanning electron microscope

Serial Number Restoration

• Serial #’s are found on engine blocks, firearms, other manufactured items

• Criminalist is called in when # is obliterated• Stamping of serial # causes a permanent strain

which extends into the metal surface (beneath the original #’s)

• Chemical treatment w/ etching agents makes obliterated #’s appear

• Etching agents dissolve faster in strained areas-but if obliteration is below zone of strain-NO HELP

Trajectory Trajectory

• Two reference points are needed to define the trajectory.

• Investigators can figure the shooter discharged the firearm somewhere along that line.

Path of bullet

Horizon

Wind shield

Distance along path of bullet to window, 23.9”

Distance along horizon to window, 23.5”

yx

60 feet

Trajectory Trajectory

• Reference points can be bullet holes in objects or victims.

• An entry point and exit point on a victim can be used.

• Gunshot residue or spent cartridge casings can be less specific reference points.

• Investigators can use lasers to trace a straight-line path to help determine the position of the shooter.

Trajectory Trajectory

Determining the Location of the Shooter Using the illustration on Slide 11 and adding that the shot came from a nearby building, these conclusions can be made:

1. Since the building is about 60 feet away, the shooter was about 11 feet above the bullet hole in the seat, which was 4 feet above the ground.

2. This height of about 15 feet off the ground puts the shooter on the second floor in that building.

Bullet Wounds Bullet Wounds 1. Why do entrance wounds tend to be smaller than

exit wounds?

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

3. Where is gunshot residue usually found?

4. If the gun is fired with the muzzle touching the victim’s skin, what telltale mark may show up?

5. Will larger or will smaller caliber bullets tend to lodge within the body rather than passing through? Why?

• Front line (18 min)

• Trayvon Martin- bullet distance intermediate -cnn (hoodie/florida 2012)

• Washington post may 18-• “A lab report, based on an examination of the two sweatshirts Martin

was wearing, found holes and gunshot residue consistent with a “contact shot,” meaning the gun was pressed against Martin’s chest. An autopsy report said that the gunshot wound indicated he was shot from an “intermediate range,” which experts say is between one and 18 inches away.”

• Mythbusters- blown away (12 gauge shot, 50 caliber shot)