Transcript Firearms

Chapter 17
Firearms
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
.45
.32
9mm
.38
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a cartridge of a fired case is withdrawn from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
Individual
• Number, width,depth • Striae impressed by
of lands and grooves
barrel
(on a bullet the lands • Striae due to wear and
will be an indentation use
the grooves a raised
• Striae due to neglect
portion)
and abuse
• twist
Cartridge Case Characteristics
Class
Individual
• Trademarks
• Firing pin impressions
(manufacturer’s name, • Breech face marks
head stamp)
• Ejector marks
• Shape (rimmed,
• Extractor marks
rimless, straight)
• Caliber
• composition
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 –w/ IR
Blood GSR
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
Wind shield
Distance along path of
bullet to window, 23.9”
Path of bullet
x
y
60 feet
Horizon
Distance along
horizon to
window, 23.5”
• Two reference points are needed to define the
trajectory.
• Investigators can figure the shooter discharged
the firearm somewhere along that line.
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 straightline path to help determine the position of the
shooter.
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
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)