ch 15 – firearms, tool marks, and other impressions techniques for rifling a barrel. class and...

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.

• 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

• 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

• 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

• 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

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• Extractor: the mechanism in a firearm by which a cartridge of a fixed case in withdrawn from the chamber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

• 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

Ch15 - Impression Evidence

Tool Marks

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

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

Toolmarks

• Any implement can be a tool for the purpose of leaving a mark– crowbars– tire irons– screwdrivers– metal pipes– heavy flashlights– fingernails

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

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

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

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

Striation Patterns

A reference mark made by a tool dragged at various angles

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

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

Tool Mark Comparison Examples

Comparison of two marks made by an axe blade.

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.

Tool Mark Comparison Examples

Matching evidence (L) and reference (R) prymarks

Impression & Print Testimony

• Essentially opinion testimony– rests heavily on experience & expertise of the

examiner

Firearms

A Brief Overview

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

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

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

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

Automatic

• Fully automatic weapons will continue to fire & expend casings as long as the trigger is depressed and ammunition available

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

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

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

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

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

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

A Colt .45

The Rifled Barrel

• Has a series of spiral cut-outs that run the length of the barrel– depressions

• grooves

– raised portions• lands

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Moving Components

• All of the moving components contact the cartridge rather than the bullet– can leave useful impressions on shotgun shell

cartridges

Ammunition

Construction

Bullet Morphology

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

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

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

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

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

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

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

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

Cartridge Cases

Cutaway photos of three kinds of cartridge cases

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

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

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

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

Compression Formed Shotgun Shell

• Pellets are separated from the propellant by one or more overpowder wads– seal the propellant

gases behind the shot

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

Impression Evidence

Bullet & Cartridge Comparisons

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

How A Bullet Acquires Rifling Impressions

Rifling Impression Patterns

Bullet comparisons

• Test bullet generated by firing into a water chamber or cotton box

• Bullets are then examined using a comparison microscope

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

Cartridge Case Comparisions

Marks which might be found on a spent cartridge

Cartridge Case Comparisons

• The hole in the center is the imprint from the primer pin

• Striation within circle are breechblock impressions

Breechblock Comparison

Evidence Cartridge Reference Cartridge

Breechblock Comparison

Breechblock Face Microscopic ComparisonNot a match

Bullet Matches

Extractor Matches

Firing Pin Match

Some Rimfire Impressions

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)

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