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Chapter 17 Firearms.

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Presentation on theme: "Chapter 17 Firearms."— Presentation transcript:

1 Chapter 17 Firearms

2 Firearms (not ballistics –study of projectiles)

3 Inside the Oakland Crime lab- ballistics Britain’s CSI School- ballistics

4 JFK Assasination (3 min in- also power point- lindbeck
Cold case JFK Nova

5 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)


7 Grooves: Rifling: Bore:
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

8 Lands: Caliber: The raised portion between grooves
Diameter of the bore (distance between opposite lands) measured in 100th of an inch or mm (Typical: .22,.32,.38,.45 or 9mm)

9 .22 .32 .38 .45 9mm

10 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)

11 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

12 Shotguns Most have smooth barrels w/ diameter in gauge (Note: can have rifled shotgun barrels) Higher the gauge the smaller the diameter

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

14 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Extractor: mechanism in a firearm by which a cartridge of a fired case is withdrawn from the chamber Ejector: mechanism in a firearm that throws the cartridge or fired case from the firearm

15 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

16 Firing a gun

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

18 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.

19 Ejector Marks Imparted to the cartridge case when it is ejected out of the weapon.

20 Extractor marks Imparted onto the cartridge case when the case is extracted from the barrel.

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

22 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)

23 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

24 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

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

26 Automated Firearm Search Systems
Allows FBI to store bullet and cartridge surface characteristics Allows a network of jurisdictions to share info

27 IBIS: Integrated Ballistic ID system
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


29 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


31 Distances Contact (gun touching victim)
Heavy concentration of GSR, scorch marks, fiber melt, blow back produces a stellate (star tear pattern)


33 12-18 inches A halo of GSR, scattered specks of unburned and partially burned powder


35 > 3 feet No GSR bullet wipe-visual indication hole; dark ring of carbon, dirt, lubricant, primer that wipes from bullet surface


37 Bullet wipe

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


40 Studying Gunpowder on Clothing
Examine microscopically If lack of color contrast or blood is present – use infrared photography Chemical Test (for nitrates) Gneiss- develops pattern Sodium Rhodizonate w/ acid GSR

41 Blood GSR Blood GSR –w/ IR
Stripped Clothing

42 Gneiss Test Sodium Rhodizonate

43 Studying Gunpowder on Hands, body
Old Test- dermal nitrate (lack of specificity- tests (+) w/ other substances

44 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


46 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


48 Trajectory Two reference points are needed to define the trajectory.
Path of bullet Horizon Wind shield Distance along path of bullet to window, 23.9” Distance along horizon to window, 23.5” y x 60 feet Two reference points are needed to define the trajectory. Investigators can figure the shooter discharged the firearm somewhere along that line.

49 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.

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

51 Bullet Wounds Why do entrance wounds tend to be smaller than exit wounds? If the bullet penetrates clothing, what can fibers embedded in the wound indicate? Where is gunshot residue usually found? If the gun is fired with the muzzle touching the victim’s skin, what telltale mark may show up? Will larger or will smaller caliber bullets tend to lodge within the body rather than passing through? Why?

52 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.”

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

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