ch 15 – firearms, tool marks, and other impressions techniques for rifling a barrel. class and...
TRANSCRIPT
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Ch 15 – Firearms, Tool marks, and Other
Impressions• Techniques for rifling a barrel.• Class and characteristics of bullets and
cartridges.• Comparison microscope for bullets and
cartridges.• Caliber Vs. gauge.• Distance of a target from a fired weapon.• Lab tests for determining an individual
has fired a weapon – limitations of present techniques.
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• Restoring an obliterated serial number.• Proper collection and preservation of
firearm evidence.• Comparing suspect’s tool to a tool
mark.• Significance of class and individual
characteristics to the comparison of impressions.
• Common field reagents for enhancing bloody footprints.
http://www.firearmsid.comhttp://www-medlib.med.utah.edu/WebPath/TUTORIAL/GUNS/GUNINTRO.htmlhttp://www.fbi.gov/hq/lab/handbook/examfire.htm
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• Firearms: a discipline mainly concerned with determining whether a bullet or cartridge was fired by a particular weapon. Not to be confused with ballistics, which is the study of a projectile in motion
• Grooves: the cut or low-lying portions between the lands in a rifled
• Rifling: the spiral grooves that are formed in the bore of a firearm barrel to impact a spin to the projectile when it is fired
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• Bore: the interior of a firearm
• Lands: the raised portion between the grooves in a rifled bore
• Caliber: the diameter of the bore of a rifled firearm. The caliber is usually expressed in hundredths of an inch or millimeters, e.g. .22 caliber and 9 mm
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• Gauge: size designation of a shotgun, originally the number of lead balls with the same diameter as the barrel that would make a pound. For example, a 12-gauge shotgun would have a bore diameter of a lead ball. 1/12 pound in weight. The only exception is the .410 shotgun, in which bore size is 0.41 inch
• Breechblock: the rear part of a firearm barrel
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• Ejector: the mechanisms in a firearm that throws they cartridge or fired case from the firearm
• Extractor: the mechanism in a firearm by which a cartridge of a fixed case in withdrawn from the chamber
• Distance determination: the process of determining the distance between the firearm and a target usually based on the distribution of powder patte3rn or the spread of a shot pattern
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• Greiss Test: a chemical test to develop patterns of gunpowder residues around bullet holes
• Choke: an interior constriction placed at or near the muzzle end of a shotgun’s barrel for the purpose of controlling shot dispersion
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Ch15 - Impression Evidence
Tool Marks
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Impression Evidence
• Any mark made in a softer surface by a harder implement or “tool”– tracks in mud– pry mark in a door frame– fingerprint in wax– bite mark– markings found on bullets & cartridge cases
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Imprints
• Tire tracks & shoe prints most common kinds of imprint evidence found in a crime lab
• Mass production of tire treads & shoe soles produces only class evidence– tread pattern
– trademark logos
• Individualizing potential comes from random characteristics acquired by use & wear
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Toolmarks
• Any implement can be a tool for the purpose of leaving a mark– crowbars– tire irons– screwdrivers– metal pipes– heavy flashlights– fingernails
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Types of Impressions
• Impressions made by striking, pressing or imprinting– leave a single point impression
• impressions made by cutting, sawing, dragging or prying– leave characteristic striations indicative of
movement across a surface
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Striation Patterns
• Pattern of striations produced by the edges of a tool can be traced back to microscopic irregularities in the sharp edges or surfaces intended for cutting
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Striation Patterns
• Mass machine manufacturing of tools causes them to be very similar when they leave the factory– the greater the wear patterns the higher the
potential for individualization
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Striation Patterns
• Tool marks are often compared with another mark made by the suspect tool rather than the tool itself– suspect tools are never fitted into the tool mark
– generation of the reference mark becomes a potential source of error
• different marks may be produced depending on the angle & amount of pressure used
• the substrate used may alter the appearance of the tool mark
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Striation Patterns
A reference mark made by a tool dragged at various angles
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Tool Mark Examination
• Magnification is usually needed to reliably compare the minutiae of marks & striations
• Comparison Microscope– Allows two samples side
by side in the same microscopic field
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Gross Bullet Comparison• Photo shows 4 different
bullets• Test (reference) & evd
(evidence) bullets have been aligned so that their land impressions match up at the dividing line.
• A greater magnification would be used for striae comparison
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Tool Mark Comparison Examples
Comparison of two marks made by an axe blade.
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Tool Mark Comparison Examples
• Comparison of a reference mark (left) to an evidence prymark left on a cash register. With larger tools and especially if the mark is on an edge, a comparison scope may not be necessary.
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Tool Mark Comparison Examples
Matching evidence (L) and reference (R) prymarks
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Impression & Print Testimony
• Essentially opinion testimony– rests heavily on experience & expertise of the
examiner
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Firearms
A Brief Overview
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Types of Firearms
• Firearms can differ in many components– the way bullets are loaded & chambered– the cocking mechanism– the firing action– the ejection of used cartridges
• A main distinction is which components are automated & which are manual
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Types of Firearms
• May be classified by their state of automation– single-shot (manual)
– semiautomatic
– automatic
• May also be classified by aspects of their physical design– Rifled
– Smoothbore
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Single-shot (Manual)
• Few weapons are operated completely manually– a single bullet being loaded by hand after each shot
• Lever-action or bolt-action weapons (rifles)– require the manual activation of a lever or turnbolt to
• extract & eject expended cartridges
• cock firing mechanism
• load a live round into the firing chamber
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Semiautomatic (Self-loading)• Use the recoil energy from a fired bullet to
– extract & eject expended cartridge casing
– to load a new round into the firing chamber
– The trigger must be activated manually for each shot fired
• Two Types (usually revolver type pistols)– Single-action
• must be manually cocked before firing
– Double-action• cocked by the pull of the trigger
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Automatic
• Fully automatic weapons will continue to fire & expend casings as long as the trigger is depressed and ammunition available
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Rifled Firearms
• Contain a set of spiraling lands (raised ridges) & grooves (complementary depressions)– give a bullet rotational motion which helps it fly
straighter & farther
• Types of rifled firearms– Pistols (handguns)
– Rifles
– Machine & submachine guns
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Pistols (Handguns)
• Designed to be fired with one hand– originally designed for cavalrymen who could only
spare one hand from the reins of the horse
• There are some single-shot pistols used for specialized target shooting
• Most pistols have revolving cylinders (revolvers)– can fire more than one shot before reloading
– many are semiautomatic
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Rifles• Designed to be fired with two hands, usually from
the shoulder• There are some small bore sporting rifles that are
single shot• Most are semiautomatic or automatic
– redirect the energy from either the bullet recoil or hot propellant gases to extract & eject the cartridge casing, then cock & load a new bullet
– Assault rifles may be used either in semi-automatic or automatic mode
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Machine & Submachine Guns
• Machine guns have a heavy recoil– designed to be fired from some type of sturdy
mounting
• Submachine guns are designed to be fired while being held in the hands
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Smoothbore Firearms
• Shotguns are the only smoothbore small arm currently in use
• Fire multiple small projectiles (pellets or shot) so accurate aiming not an issue – barrels smooth rather than rifled– barrels shaped or indented to control the spread
of the shot as it exits the muzzle• choke
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Shotguns
• May be single or double-barreled– allows the two muzzles to be set for delivery of
the ammunition at different distances or spread patterns
• Repeating shotguns may be reloaded – manually with a pump action or bolt action– semiautomatically using the same mechanisms
as a semiautomatic rifle
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A Colt .45
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The Rifled Barrel
• Has a series of spiral cut-outs that run the length of the barrel– depressions
• grooves
– raised portions• lands
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Rifled Barrel
• The # of lands & grooves; their direction & rate of twist are characteristic of a particular product from a specific manufacturer– class characteristics
• Examples– .32 caliber Smith & Wesson revolvers
• 5 lands & grooves twisting to the right
– .32 caliber Colt• 6 lands & grooves twisting to the left
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Barrel Manufacture
• Before 1940, grooves cut one at a time by drawing a scraper repeatedly down the inside of the barrel as it was rotated
• Three modern methods of rifling– broaching– button rifling– hammer forging
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Broaching
• The diameter of the hole reamed in the steel will be the final diameter (caliber) of the barrel
• Grooves cut with a tool called a gang broach– consists of a series of circular cutting tools with
projecting teeth positioned a regular intervals on a rod
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Broaching
• The teeth cut away the metal in the grooves as the broach is forced through the barrel by hydraulic pressure
• Successive cutting disks increase in diameter until the desired groove depth is reached
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Button Rifling (Swagging)
• The diameter of the original hole is drilled smaller than the final desired caliber
• Rifling button tool bears an exact negative impression of the lands & grooves needed
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Button Rifling (Swagging)
• Tool is forced through the reamed hole simultaneously rifling & expanding the barrel
• No actual cutting of metals involved• .22 caliber rifles are commonly made this way
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Hammer Forging
• The hole is drilled slightly larger than the final desired caliber
• Barrel is hammered of a specially shaped mandrel to produce the rifling characteristics
• The mandrel is forced out the end of the barrel
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Bore Diameter• All firearms may be characterized by their bore
diameter or caliber– the measure of the diameter in hundredths of an inch or
in millimeters• American & British weapons are normally given in hundredths
of an inch (.22; .45)
• other countries are normally in mm (9 mm)
• The larger the number, the larger the diameter– .22 is smaller than .45
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Bore Diameter of Rifled Barrels
• Measured from the tops of opposing lands when there is an even number of lands & grooves
• measured from a circle tangent to the tops of the lands if there is an odd number of lands & grooves
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Shotguns
• Typically characterized by their gauge• Gauge is a term which comes from the days
when all firearms fired spherical lead balls– gauges were expressed as the number of
appropriately-sized bullets that could be made from one pound of lead
• 10-gauge shotgun– 10 lead balls of about the same diameter as the barrel
could be made from a pound of lead
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Shotgun Gauge
• The higher the gauge number, the smaller the diameter of the barrel– 12-gauge shotgun has a bore diameter of 0.730
inches– 16-gauge shotgun has a bore diameter of 0.670
inches
• Exception– .410-gauge shotgun has a barrel 0.410 inches in
diameter
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Moving Components
• Some components of a firearm’s firing & cartridge ejection mechanism are of importance to the forensic examiner– the breechblock– the firing pin – the extractor– the ejector
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Breechblock• Supports base of the
cartridge in the chamber
• Often finished by hand filing– characteristic striations on
this part contact the back of the cartridge leaving individualizing markings on the cartridge base
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Firing Pin (Primer Pin)• Strikes the cartridge
primer to initiate the firing process
• Finished on a lathe or filing by hand
• Can transfer striations to the soft metal of the primer cap
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Extractor & Ejector
• Extractor– extracts a spent cartridge from the chamber
• Ejector– ejects the extracted cartridge from the side of
the weapon body
• Metal parts of these mechanisms can leave individualizing markings on cartridge cases
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Moving Components
• All of the moving components contact the cartridge rather than the bullet– can leave useful impressions on shotgun shell
cartridges
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Ammunition
Construction
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Bullet Morphology
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Bullets
• When black powder was used as a propellant– projectiles had relatively low velocities– bullets were usually almost pure lead
• With smokeless powders– burn rate & heat production much greater– pure lead bullets foul rifling
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Common Types of Bullets For Rifled Firearms
• Lead Alloy– lead hardened by addition of antimony
• Semijacketed– have a copper-alloy or aluminum jacket that
covers part of the bullet surface• usually nose is left exposed
– nose configurations encourage penetration & expansion on impact
» soft-point, hollow-point, bronze point, etc
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Common Types of Bullets For Rifled Firearms• Full jacketed
– consist of a lead core surrounded by a jacket of copper-nickel alloy or mild steel
– usually core is left exposed at the base of the bullet– Semiautomatic pistols use full jacketed bullets
• noses of bullets must slide up a ramp when rounds chambered
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Bullet Shapes
• Influence aerodynamic properties– round 0r pointed noses reduce air resistance
• increases striking power
– some have boattailed bases• act as a rudder to reduce turbulance & decrease drag
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Cannelures
• An important aspect of bullet morphology is the presence or absence or cannelures– circumferential grooved rolled into the surface
of a bullet• can contain lubricant
• can serve as seats for the crimped mouths of cartridge cases
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Primers
• Since gunpowder is relatively stable, it I ignited via a more flammable or explosive primer
• Types of systems– rimfire cartridge
• primer placed in the rolled rim of the cartridge casing
– centerfire cartridge
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Boxer Primer System
• Consists of a metal cup which holds primer.
• A small anvil rests on top of primer material
• Firing pin detonates the explosive between cup & anvil
• Flame ignites propellent through a single flash hole
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Berdan Primer System
• Incorporates a conical anvil as part of the base
• Primer cap is a small metal up containing a pellet of primer compound
• 2 or 3 small flash holes communicate the flame from primer
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Cartridge Cases
Cutaway photos of three kinds of cartridge cases
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Cartridges Cases
• Made from brass– may be nickel-plated to prevent corrosion
• Come in a variety of different shapes– Revolvers fire straight rimmed cartridges
– Self-loading pistols fire straight rimless cartridges
• Can have cannelures rolled into them to prevent the bullet from being pushed back into the case
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Cartridge Cases
• Bullets may be held in place by criming or staking the mouth of the cartridge case into a cannelure on the bullet
• May have nformation stamped on the head of the cartridge– identify manufacturer– identify caliber
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Shotgun Shells
• Most contain pellets which come in a variety of sizes– the smaller the number, the larger the pellet
• 000 buckshot pellets are 0.36 inches in diameter
• No. 12 birdshot pellets are 0.05 inches in diameter
• The load in each shotshel depends on the gauge of the shell & the size of the pellets– 12-gauge No. 1 buckshot contain 16 pellets
– 16-gauge no. 1 buckshot contains 12 pellets
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Shot gun Shells
• Shotgun pellets may be made of pure lead (drop shot), lead alloy (chilled shot) or soft steel
• Smaller size pellets are manufactured by pouring molten metal through a perforated plate & allowing it to fall some distance– solidifies as spheres
• Larger size are made by pressing lengths of wire between cup-shaped dies
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Compression Formed Shotgun Shell
• Pellets are separated from the propellant by one or more overpowder wads– seal the propellant
gases behind the shot
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Firing A Shot
Chambered bulletFiring pin hits primer
& sending a flash to ignite the powder
Bullet is propelled forward though the gun barrel as the spent cartridge case is slammed back against breechblock
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Impression Evidence
Bullet & Cartridge Comparisons
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Interior View of a Rifled Gun Barrel
• Horizontal striations on the grooves (G) come from the rifling process
• Vertical striations on the lands (L) come from the initial reaming of the barrel
• These markings impress the bullet as it travels through the barrel
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How A Bullet Acquires Rifling Impressions
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Rifling Impression Patterns
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Bullet comparisons
• Test bullet generated by firing into a water chamber or cotton box
• Bullets are then examined using a comparison microscope
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Bullet Comparisons• One bullet is rotated until a
well-defined land or groove comes into view
• The other bullet is then rotated in search of a matching region
• Analyst must allow for distortion between bullets due to mutilation on impact
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Cartridge Case Comparisions
Marks which might be found on a spent cartridge
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Cartridge Case Comparisons
• The hole in the center is the imprint from the primer pin
• Striation within circle are breechblock impressions
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Breechblock Comparison
Evidence Cartridge Reference Cartridge
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Breechblock Comparison
Breechblock Face Microscopic ComparisonNot a match
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Bullet Matches
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Extractor Matches
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Firing Pin Match
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Some Rimfire Impressions
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Automated Firearm Search Systems
• National computerized networks have been developed to allow labs to search each other’s data
• DRUGFIRE (FBI)– concentrates on cartridge markings although
bullet striae can be stored as well
• IBIS (Integrated Ballistic Identification System)
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Automated Firearm Search Systems
• IBIS (Integrated Ballistic Identification System) – developed by the Bureau of Alcohol Tobacco &
Firearms (ATF)– incorporates two separate software programs
• Bulletproof– for bullet-specific markings
• Brasscatcher– for cartridge cases