Firearm Identification in the Forensic Science Laboratory

F I R E A R M

I D E N T I F I C AT I O N

By Robert M. Thompson

Christopher Chiles

President

Scott Burns

Executive Director

John Wilkinson

Program Manager, Gun Violence Prosecution Program

2010 by the National District Attorneys Association

This project was supported by the Bureau of Justice Assistance under grant number

2008-MU-MU-K004 awarded to the National District Attorneys Association. The Bureau

of Justice Assistance is a component of the U.S. Department of Justice’s Office of Jus-

tice Programs, which also includes the Bureau of Justice Statistics, the National Insti-

tute of Justice, the office of Juvenile Justice and Delinquency Prevention, and the Office

for Victims of Crime. Points of view or opinions in this document do not necessarily rep-

resent the official positions or policies of the U.S. Department of Justice of the National

District Attorneys Association.

GUN VIOLENCE PROSECUTION PROGRAM

A program of the National District Attorneys Association

44 Canal Center Plaza, Suite 110

Alexandria, VA 22314

www.ndaa.org

F I R E A R M

I D E N T I F I C AT I O N

I N T H E F O R E N S I C

S C I E N C E L A B O R AT O R Y

By Robert M. Thompson

Program Manager for Forensic Data SystemsOffice of Law Enforcement StandardsNational Institute of Standards and Technology

7 Introduction

9 The Science of Firearm Identification

12 The Production of Firearm Toolmarks on the Fired Cartridge

26 The Examination Process and Trial Preparation

31 Appendix and Glossary

CONTENTS

FOR A PROSECUTOR to be successful, heor she must be cognizant of the expectations oftoday’s jury. Thanks to the modern electronicmedia, use of the forensic sciences has caughtthe imagination of the public, and the potentialjury pool has demonstrated that it has certainexpectations when a case is brought before it.No matter how fantastic or erroneous these ex-pectations are, practitioners in law enforcementand experts in the forensic sciences have to dealwith them in a forthright manner. The beststrategy is for the prosecutor to be well ac-quainted with the capabilities and limits of theforensic science disciplines that may be thelinchpin in the investigation and, more impor-tantly, in the prosecution of a defendant at trial.

This monograph serves to introduce theprosecutor to the principal elements of one ofthe forensic specialties, the science of “firearmand toolmark identification.” Many of thewords and terms printed in bold in the text aredefined in the glossary. The monograph pro-vides an introductory discussion of the specialtyof toolmark identification when the tool in-volved is a firearm. The tool surfaces repre-sented here involve one or more of the

following: the interior of the barrel, the cham-ber, parts of the action, and ammunition mag-azine components. These surfaces of the firearmcan produce toolmarks on fired and unfiredammunition components. The forensic scientistviews a “tool” as the harder of two objectswhere the surface of the harder “tool” producestoolmarks on a softer material. For example, thetool surface of the hard barrel interior leavestoolmarks on the softer metal of the fired bul-let. Another example is when a cartridge is firedin a firearm. The softer metal used in the car-tridge case construction may show toolmarkscaused by the interior chamber and action sur-faces coming in contact with the cartridge case.The action is the firearm’s loading and firingmechanism.

For there to be a potential for toolmark iden-tification, the tool working surface (1) musthave individuality, and (2) the toolmarks mustbe reproducible for comparisons. If it is deter-mined that the individual character of the toolworking surface is reproduced in the toolmarksfrom repetitive markings, an examiner may beable to make an identification in later compar-isons.

INTRODUCTION

Editor’s Note: Robert M. Thompson is the program manager for Forensic Data Systems in the Office of Law Enforcement Stan-dards at the National Institute of Standards and Technology (NIST). Prior to working at NIST, Mr. Thompson was a senior firearmand toolmark examiner for the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) Forensic Science Laboratory in Am-mendale, Maryland. Mr. Thompson was an ATF examiner for 14 years, also working in the San Francisco, California ATFForensic Laboratory. He worked as a forensic scientist for 15 years prior to joining the ATF.

FIREARM IDENTIF ICATION IN THE FORENSIC SCIENCE LABORATORY 7

THE HISTORY of the science of forensic firearm(and toolmark) identification, and its court acceptance,spans over 100 years in the United States. The princi-ples and the primary tools used in the science havechanged very little during this time. The comparisonmicroscope, the primary tool used by the profession,has not changed in its basic design for almost 80 years.Before this instrument became available, examiners re-lied on photomicrograph comparisons to determineidentity of fired bullets or cartridge cases, which was atime consuming and laborious method. (The terms car-tridge case, casing, and case will be interchangeable inusage.) With the engineering of the “optical bridge,”two compound microscopes were joined together, giv-ing the examiner the ability to observe and comparetwo objects at the same time under magnification. Thegenesis of the modern comparison microscope was ac-celerated with the addition of microscope stages thatwere designed for the mounting of fired bullets, cases,and other items bearing toolmarks. The science offirearm identification was soon propelled forward inforensic investigations in this nation and worldwide.

Today, firearm units in crime laboratories might useother complimentary microscopic and photographicinstrumentation, but for matters concerning the iden-tification of toolmarks on fired bullets, cases or anyother object, the comparison microscope is an ab-solutely necessary instrument.

The recent computer technology for searching imagedatabases for presumptive linkages is demonstrated bythe installation and use of the National Integrated Bal-listic Information Network (NIBIN). NIBIN has a firmfoundation of acceptance in the forensic science com-munity and the courts.

As was described previously, the forensic science offirearm identification is a specialized sub-specialty oftoolmark identification specifically related to thefirearm mechanism’s working surfaces. The firearm ismade up of a number of tools, many of which come

into contact with, and leave toolmarks on the softermetal of the cartridge case and/or bullet. The firearm,as with any other tool, has features that were designedby the factory. Features that are determined by the man-ufacturer include the size of the cartridge chamberedby the firearm, the orientation of the extractor andejector, and the number, width and twist direction ofthe land and grooves of the barrel rifling. These char-acteristics can be imparted as toolmarks on the firedbullet and case during firing, and can be classified bytheir class characteristics. These class characteristics aretypically the first classification of toolmark evidence thatthe examiner seeks in the examination. Class charac-teristics help narrow the population of potential firearmsources. The following experience common to us allgives an example of sorting using class characteristics:

You are leaving a store and have to find yourcar in a large, crowded parking lot. You begin bylooking for a certain vehicle type (SUV, convert-ible, sedan, etc.), make (Ford, Chevy, Volvo, etc),

A comparison microscope

8 FIREARM IDENTIF ICATION IN THE FORENSIC SCIENCE LABORATORY

THE SCIENCE OFFIREARM IDENTIFICATION

model, and color. You are looking for the classcharacteristics of your car.

If the class characteristics agree in every respect withthe evidence item (i.e., the cartridge case or the recov-ered bullet) and with the test-fires from a suspectfirearm, the examiner then uses the comparison micro-scope to compare the individual characteristics ofboth evidence and test toolmarks. Individual character-istics are random in nature, usually arising from the toolworking surface incidental to manufacture, but can alsobe the result of use, wear, and possible care and/or abuseof the tool.

Building on the example of finding your carin the parking lot, you find what appears to beyour car, but you know it is a popular model andhave seen very similar cars in other parking lotsand on the road. So you approach the car thatlooks like yours and you search for those individ-ual characteristics that make it your own. For ex-ample, you look at the license plate, windowdecals, dings and dents. These all together confirmthat the car is yours and not someone else’s. You donot search for every individual feature that youknow is on your vehicle, but just enough to deter-mine its identity.

The characteristics that make the tool surface uniqueare called individual characteristics. When these char-acteristics are compared in toolmarks, and sufficientagreement is found, an identification can be established.These characteristics are from imperfections on the toolsurface that make the toolmark. The imperfections, typ-ically microscopic, usually arise during the tool manu-facturing process. In addition, the surface may also gainimperfections and irregularities through use, wear, cor-rosion, and damage. Remember your car in the park-ing lot? Individual characteristics would be the dings

and dents, the license tag, the rust spots, and the wind-shield crack. As you can see, these characteristics wouldbe acquired over time compared to the few that wouldbe seen on a new show room car.

There is a toolmark classification termed subclasscharacteristics, sometimes referred to as “carryover.”These tool surface characteristics are incidental to man-ufacture, are significant in that they relate to a subgroupfrom which they belong, and arise from a tool sourcethat can change over time. Subclass characteristics canbe reproduced on a limited number of tools. Therefore,the examiner cannot base identification on toolmarksderived from such a source knowing there is a goodchance that such a toolmark could originate from sev-eral firearm barrels.

A source of such a subclass characteristic may be pro-duced during the cutting of barrel grooves in rifled bar-rels, if, for example, during the cutting of many barrelson an assembly line, one of the cutters develops a largechip that is not noticed by the machinist or quality con-trol experts. The chip on the cutter may produce acoarse imperfection in an otherwise cleanly cut groove.The detail from this defect may be reproduced on anumber of consecutive barrels (i.e. carried over), untilthe cutting tool is discarded or re-sharpened. The ex-perienced firearm examiner is aware of such artifactsoccurring in the barrel forming process, and under-stands that these types of coarse, continuous toolmarks,while useful in the examination and comparisonprocess, cannot be a basis for an identification. This isone of a number of instances in firearm and toolmarkcomparisons in which subclass characteristics have tobe considered before an identification of a toolmarksource is concluded.

Toolmarks generally appear in two forms: striatedand impressed. Striated toolmarks are formed when atool-working surface is placed on another surface andmoved parallel to that surface. In other words, a toolmakes a scratch or scrape mark on the surface of an-

FIREARM IDENTIF ICATION IN THE FORENSIC SCIENCE LABORATORY 9

other object. The detail in this toolmark has the ap-pearance of parallel lines, called stria. Under the micro-scope the stria are seen as a profile consisting of hills,valleys and ridges. If the stria is very shallow, the tool-mark appears as a pattern of lines. Impressed toolmarksare formed when the tool surface is forced perpendic-ularly to another surface. This toolmark has the appear-ance of having been stamped. Due to the process ofimpressing a toolmark, stria production is very limited,and may not be formed at all. Instead, the tool workingsurface imperfections give the negative detail in thetoolmark. The examiner uses a comparison microscopeto determine identification for both striated toolmarksand impressed toolmarks.

An example of a striated toolmark is the actionthat occurs with a car’s windshield wiper against awet windshield. If the wiper is well worn, nicks ofvarious sizes will be randomly present on theblade. When the wiper is used on the windshield,a pattern of lines is drawn across the arc of thewiper movement. The placement of these imper-fections cannot be accidentally duplicated on anyother blade length, and the pattern of stria is indi-vidual to that particular blade. Additionally, thetoolmark in the windshield is duplicated on eachstroke, exemplifying the reproducibility of thetoolmark.

Similarly, the impressed toolmark can becharacterized by the stamping of coins. The toolthat impressed the coin had the negative profile ofthe coin. The coin produced has the impression ofthe tool on its softer metal. The class characteris-tics of a 2004 dime are visually apparent. However,microscopic imperfections impressed on the dimemay be used to identify which specific tool in themint was used to produce the coin.

For the science of toolmark identification, the un-derlying hypothesis is that a toolmark can be identifiedto a specific tool that produced it, to the practical exclu-

sion of all other tools. Clearly, it is impossible to provethis hypothesis by testing all tools ever produced in theworld. Instead, identification must be inferred, based onobservation and experimentation. Over many years sci-entists have documented that the surfaces of tools thatmake toolmarks are microscopically dissimilar and in-dividual in nature. This dissimilarity is observed and po-tentially quantifiable in what is called “knownnon-match” comparisons. While there is a potential forrandom agreement to a small extent, this agreementdoes not reach the quality and quantity shown betweentoolmarks made by the same tool working surface, or“known matches.” Therefore, if the agreement of tool-marks is of sufficient quality and quantity that is ex-pected from one tool, and greater in quality andquantity than has been demonstrated by the best“known non-match” toolmarks from different tools, anidentification can be made between the two toolmarks.However, as stated before, prior to the determinationof identification, the influence of sub-class characteris-tics has to be eliminated. The human being is very cog-nitive of the environment, and one of the hallmarks ofhuman reason is the detection of patterns, whether bythe senses, or by circumstances in time.

Consider your drive to and from work. Even inbusy traffic, you as an experienced driver, tune outmany of the circumstances of a routine commute.Each drive is different and has random circum-stances that vary to a degree over the weeks of thesame commute. One day you see a new sports carthat catches your eye because you appreciate sportscars. As it passes by you note its color, its lines, andperhaps the wheels. As the car recedes from sight,you reestablish the mental monotony of your trip,and in a few minutes you see an identical newsports car pass you on the road. Now, you are re-ally interested, because this rare event just hap-pened in one trip in a few minutes. You carefullycompare car number two with the mental notesyou made of car number one. You know that this

1 0 FIREARM IDENTIF ICATION IN THE FORENSIC SCIENCE LABORATORY

may be coincidence, the cars may not be producedwith many options, and it may be rare to see twocars in such close proximity. It is an interesting co-incidence (a non-match). Consider what you startto realize when you pass another and another—anumber of similar cars. You now know that some-thing special might be happening based on the co-incidental discovery of these cars in such a closespace of time. Something else must be going onto make this a singular event. Perhaps there was acar club, a manufacturer test market, or an autoshow. Any idea that this event happened simply bychance is quickly dismissed, with confidence (amatch).

The largest organization that supports the interchangeof scientific information concerning firearm and tool-mark science is the Association of Firearm and Tool-mark Examiners (AFTE), which publishes the AFTEJournal. The AFTE Journal is peer-reviewed by an edi-torial committee, with a section in each issue set asidefor responses by the readers. Peer review helps ensurethat open discussion among practitioners is maintainedand that any information being disseminated is accu-rate and reliable.

In 1992, AFTE adopted the “Theory of Identification”which reads:

1. The theory of identification as it pertains to thecomparison of toolmarks enables opinions ofcommon origin to be made when the uniquesurface contours of two toolmarks are in “suffi-cient agreement.”

2. This “sufficient agreement” is related to the sig-nificant duplication of random toolmarks as ev-idenced by a pattern or combination of patternsof surface contours. Significance is determinedby the comparative examination of two or moresets of surface contour patterns comprised of in-dividual peaks, ridges and furrows. Specifically,

the relative height or depth, width, curvatureand spatial relationship of the individual peaks,ridges and furrows within one set of surfacecontours are defined and compared to the cor-responding features in the second set of surfacecontours. Agreement is significant when it ex-ceeds the best agreement demonstrated betweentoolmarks known to have been produced by dif-ferent tools and is consistent with agreementdemonstrated by toolmarks known to have beenproduced by the same tool. The statement that“sufficient agreement” exists between two tool-marks means that the agreement is of a quantityand quality that the likelihood of another toolmaking the mark is so remote as to be consid-ered a practical impossibility.

3. Currently the interpretation of individualiza-tion/identification is subjective in nature,founded on scientific principles and based onthe examiner’s training and experience.

As part of the standardization of terms and conclusionsfor the firearms examiner to employ, AFTE developeda range of conclusions based on the Theory of Identi-fication. The examiner would conservatively describeobjective observations and the results of examinations,as follows:

Identification: Agreement of a combination of individual characteris-tics and all discernible class characteristics where theextent of agreement exceeds that which can occur inthe comparison of toolmarks made by different toolsand is consistent with the agreement demonstrated bytoolmarks known to have been produced by the sametool.

Inconclusive:a. Some agreement of individual characteristics and all

discernible class characteristics, but insufficient for anidentification.

FIREARM IDENTIF ICATION IN THE FORENSIC SCIENCE LABORATORY 1 1

b. Agreement of all discernible class characteristicswithout agreement or disagreement of individualcharacteristics due to an absence, insufficiency, or lackof reproducibility.

c. Agreement of all discernible class characteristics anddisagreement of individual characteristics, but insuf-ficient for an elimination.

Elimination:Significant disagreement of discernible class character-istics and/or individual characteristics.

Unsuitable:Unsuitable for examination.

The Production of FirearmToolmarks on the Fired Cartridge

When toolmarks are made on the fired bullet andcartridge case, their general appearance and orientationoriginate from the class characteristics of the firearmproducing those marks. Routinely, when a firearm isnot collected as part of an investigation, the firearm ex-aminer measures and characterizes the marks (both stri-ated and impressed) found on the bullet and/orcartridge case. Then the examiner compares the obser-vations and data to reference literature and databases,and produces a list of possible firearm manufacturersand possibly models, that could be the source of the ev-idence. While such a list is not all inclusive of all possi-ble manufacturers, it may be an aid in an investigationwhere a suspect firearm was not recovered. However, ifa suspect firearm is recovered, the firearms examinerwill determine if the firearm has the correct class char-acteristics by examination and test firing, and then ifthe class characteristics agree, will microscopically com-pare the test-fired bullets and cartridge cases to the ex-hibits collected in the investigation.

To better understand the placement of toolmarks onfired cartridge components, an understanding of firearmtypes, actions, ammunition, and firearm toolmark pro-ducing surfaces is necessary.

Firearm TypesThe basic types of firearms are handguns and shoulderarms. Handguns are designed to be fired by one handwithout support from the body. A shoulder arm is de-signed with a stock to be fired while being supportedby the shoulder.

• Handguns

Pistol—A firearm that has a chamber as part of the bar-rel and is typical of semi-automatic handguns.


Semiautomatic 9mm pistol

Double-action 44 Magnum Revolver with Scope

Revolver—A firearm that has a number of chambers ina cylinder that rotates on an axis; during successive fir-ing, a chamber rotates and aligns with the barrel.

• Shoulder Arms

Rifle—A firearm that has a rifled barrel and is designedto be fired from the shoulder

Shotgun—A smooth bore barreled shoulder firearm de-signed to fire shotshells that contain numerous pellets,or a single projectile.

Modern Firearm ActionsFirearms have loading and firing mechanisms called

actions. Modern firearms may have differing actionsdepending on the design of the firearm. The most com-mon forms are semiautomatic, automatic (also known asfull auto or machine gun), revolver, lever, slide (orpump), and bolt actions. A “firing cycle” is composedof the actions performed by the shooter and the firearm

mechanism to fire a cartridge, with the subsequentreadying of the firearm for a discharge of the next car-tridge. The most commonly encountered firearm ac-tions are:

• Semiautomatic—A firearm that requires a separate pullof the trigger for each shot, and uses energy from thedischarge to perform a portion of the operation orfiring cycle, usually the extraction and loading por-tions.

• Automatic—A firearm that feeds cartridges, fires, ex-tracts and ejects cartridge cases continuously for aslong as the trigger is fully depressed and there are car-tridges in the feed system.

• Revolver—A firearm that has a number of chambers ina cylinder that rotates on an axis; during successive fir-ing, a chamber rotates and aligns with the barrel.

• Lever—A firearm wherein the breech mechanism iscycled by an external lever generally below the re-ceiver.

• Slide (pump)—A firearm with a movable forearm thatis moved in line with the barrel by the shooter. Thismotion is connected to the breech bolt assembly,which performs the functions of the firing cycle thatis assigned to it.

• Bolt—A firearm where the breech closure is in linewith the barrel; the closure manually reciprocates toload, unload, and cock; and locks in place by breechbolt lugs on the bolt engaging the receiver.

Ammunition Construction,Terminology, and Nomenclature

It is common today to hear or read the term “bullet”misused in the media and in television shows andmovies. For example, a suspect was arrested with “bul-lets” in his pocket, a semiautomatic rifle that can carrymany “bullets,” or a cowboy in a shootout is “out ofbullets.” However, the unit of ammunition is properlytermed a cartridge. The cartridge consists of a case, aprimer, propellant (powder), and one or more projec-tiles. The projectile is the true bullet. (In some areas of


Lever-action Rifle

Pump-action shotgun

the United States the bullet may also be termed “pel-let.”) Experienced shooters or otherwise informed ju-rors will be very aware of the distinction betweencartridge and bullet.

Cartridges come in many sizes, shapes, and bullet de-signs. Two types of ignition systems for the modern

metallic cartridge are rim fire and center fire. Rim firecartridges, common with .22 calibers have the primercompound inside the rim of the case head. The primercompound is shock sensitive and emits a hot jet of flameonto the powder when the case rim is struck by the fir-ing pin of the firearm, similar to a toy cap being struck.Center fire cartridges have a separate primer seated inthe center portion of the case head. When the primeris struck, the jet of flame passes from the primer throughan internal opening in the bottom of the case called theflash hole, thereby igniting the powder.


Center Fire Cartridges

Center Fire Cartridge

22 Caliber Rimfire

Revolver Cartridge Lead Hollow Point Bullet

Toolmarks on Fired AmmunitionComponents and Their Sources

Fired BulletsFired bullets have impressed and striated toolmarks

that are generated by the tool working surface of therifled bore of the barrel. Rifling is the construction ofhelical grooves in the bore that impart a rotary motionor spin to a fired bullet, thereby giving the bullet morerange, stability, and accuracy. When the powder in thecartridge starts burning after ignition, the extreme pres-sure produced by the gasses causes the rear of the bul-let to deform slightly and swell to fill the inside of thebarrel. The bullet deformation helps seal the gasses be-hind it as it travels down the barrel. This bullet defor-mation effect is called obturation. The sides of thebullet are engaged by the rifling, and the soft metal is

impressed by the raised portion of the rifling calledlands and alternately, may fill in the rifling between thelands called grooves. As the bullet travels down the bar-rel, the soft metal on the sides are engraved by the ri-fling until it leaves the barrel. Some of the classcharacteristics found on a fired bullet are (1) the caliberof the bullet (diameter), (2) the number of lands andgrooves, (3) the twist of the rifling (left or right), and (4)the widths of the land and groove impressions. The abil-ity to determine all or some of a fired bullet’s class char-acteristics may be limited due to the condition of thebullet when it was recovered.


A fired cartridge case and a fired bullet. Note thefiring pin impression on the primer of the car-tridge case and the land and groove engravingon the bullet.

A fired bullet with barrel rifling impressions onthe sides.

A bullet jacket, typically found in casework.

Barrel Manufacture Methods and the Basisfor Identification of Fired Bullets

There are, and have been, numerous manufacturers offirearms with rifled barrels. Many manufacturers makerifled barrels as their main product. Each manufacturerproduces rifled barrels in a particular manner best suitedto the company’s needs. However, they all use basic pro-duction methods to manufacture rifled barrels.

Briefly, the basic steps to make a rifle barrel from alength of steel bar stock are:• The barrel is drilled lengthwise with a tool called a

deep hole gun drill. This produces a hole, which atthis stage, is not adequately smooth or sized to thespecified dimension of the designed final bore size.

• A cutting tool called a “reamer” finishes the bore byremoving coarse material from the hole drillingprocess, and perfects a true circular hole. After this ac-tion, the bore is now the proper size, relativelysmooth, and consistent dimensionally down thelength of the bore. The reamer leaves fine annular ringtoolmarks that are close to perpendicular to the boreaxis.

• The rifling in the bore may be produced by one ofthe following methods:

• A cutting tool that cuts grooves singularly, oras a “gang” where multiple grooves are cut to

the desired depth. The grooves cut one at atime are made by a hook cutter, two at a timewith a scrape cutter. Multiple grooves cut inone pass are made by a gang broach. Of thesemethods, the gang broach is commonly usedtoday for cut rifling.

• Button rifling (also termed button swage): Inthis method a very hard tungsten buttonwhich has the reverse cross section of the de-sired rifling is pushed or pulled through abore that has a smaller diameter than the but-ton. Under high pressure, the metal flowsaround the button surface as it passes downthe barrel. The rifling is “ironed in” to thebarrel interior and no metal is removed.

• Hammer forging: (In some ways may beimagined as the reverse of swaging.) A man-drel with the cross section of the rifled boreinterior is placed in a slightly larger barrelbore. A system of large hammers, undertremendous force, pound from the outside ofthe barrel onto the mandrel inside, much likea blacksmith hammers a red-hot horseshoeinto shape. The finished bore will have theimprint of the mandrel’s rifling impressed onthe interior.


The flattened side of a fired bullet that rico-cheted prior to entering the victim’s body.

Broach Cut 6-Left

Each of these rifling processes has a number of im-portant steps. The tools cutting and forming the riflingundergo change as the products are formed in the man-ufacturing process and wear down during the lifetimeof the tool. If a tool becomes too dull, or does not per-form to tolerance, then it must be sharpened, recondi-tioned, or replaced. At the microscopic level, the toolworking surface—the barrel—has its own individual-ity. That individuality can be reproduced in the en-graved toolmarks on the fired bullet. Based on thisindividuality of the interior of the barrel, bullets can beidentified to a particular barrel.


Barrel rifling

A button swage used to form rifling in a barrel.

A gang broach that progressively cuts rifling ina barrel.

Cross section of a barrel showing toolmarksthat survive the rifling procedure.

Fired Cartridge Cases

Breech/bolt Face and Firing Pin Toolmark In-dividuality on Fired Cartridge Cases

Fired cartridge cases are often left at shooting scenesbecause the shooters are not inclined to waste timesearching for the ejected and fired cartridge cases. Afired case may have a number of surfaces that bear bothimpressed and striated toolmarks from the firearmmechanism that fired it. As with bullets, cartridge casescan also bear class characteristics of the firearm that mayprovide the examiner with information needed to as-semble a list of firearm manufacturers in the event thefirearm itself is unavailable for comparison.


Microscopic comparison of a fragment of bullet jacket (left) and a test fired bullet from a suspect firearm (right).

When the firing pin or striker impacts the cartridgeprimer, it leaves an impressed toolmark on the softmetal of the primer, and any microscopic imperfectionson the surface of the firing pin can be transferred ontothe primer. These toolmarks are usually individual innature and can be reproduced during firings.


Microscopic comparison of the two firing pin impressions.

Firing pin impression comparison of two fired.22 caliber cartridge cases.

Breech face and firing pin. The extractor is inthe upper left, 9 to 12 o’clock.

As the powder burns and creates pressure, the case swellsinside the chamber and seals the gasses from escaping,except down the barrel behind the bullet. This sealingeffect, as described with fired bullets, is also called ob-turation. The softer metal of the case (brass, aluminum,soft steel) may receive toolmark impressions from thechamber sides called chamber marks. As the bulletpasses down and out of the barrel, the head of the caseimpacts the breech or bolt face that holds the case in thechamber. The imperfections of the breech face impressa negative impression on the case head and are calledbreech face marks.


Microscopic comparison of the detail found in the firing pin impressions.

Microscopic comparison of chamber marks on thesides of two cartridge cases.


Microscopic comparison of the breech face detail on two cartridge cases.

The same fired cases as previously shown, microscopically compared side by side.

Ejector marks An ejector is a firearm part that assists in the removal

of a fired case from the firearm. This ejection processclears the chamber area for subsequent loading of an-other cartridge into the chamber. The ejector is typi-cally attached to the frame, remains stationary, and kicksthe case out of the ejection port after chamber extrac-tion. If there is enough force in this event, the case willhave an impression of the ejector, and this toolmarkmay be identifiable to a particular ejector. (Not all ejec-

tors are of this design. Some are integral to the bolt orfiring pin.

Action Marks on Fired Cartridge Cases andUnfired Cartridges; Investigation Potentials

A number of firearm tool surfaces may leave markson the cartridge case when a cartridge is fired in afirearm. Toolmarks can be produced when a cartridge isloaded, chambered, and extracted without a discharge.Take for example a semiautomatic pistol. The ammuni-tion magazine may leave toolmarks on the side of thecases when the cartridges come in contact with the

magazine lips. The cartridges in the magazine are underspring tension and are held in place by magazine lips.The lips may scrape the sides of each case as they arepushed into a chamber, or as they are loaded into, orremoved from, the magazine by hand. These toolmarkson the cases may be produced while the magazine isunattached to the firearm. If there is sufficient individ-ualizing detail in these marks (which can be very lim-ited), an identification to a particular magazine may beestablished. This is important to an investigator becausea magazine left at the scene, or confiscated from a sus-pect, may be compared to ammunition or fired casesrecovered at the scene, or ammunition that is seized inthe course of the investigation, even when the firearmis not recovered.


Microscopic comparison of ejector impressions ontwo cartridge cases.

A cartridge in an ammunition magazine. The topcartridge held in place by the two magazine lips.


The side view of an ammunition magazine with theorientation of the cartridges that are to be inserted.

The nearly horizontal arching toolmarks on the sides of the two cartridge cases were made by a lip of anammunition magazine.


A view through the ejection port of a pistol. To the left is the pistol slide and breech face. In the center of thebreech face is the firing pin aperture. To the left of the breech face is the extractor. To the lower right of thebreech face is the ejector. Both magazine lips can be seen below and forward of the slide.

Similarly, cartridges may be loaded into, and ex-tracted from, a firearm chamber without firing. A toolthat helps this process is called an extractor, which isfound on the bolt or slide of the firearm. The tool re-sembles a claw, which grabs the case head at the base ofthe cartridge, and may produce scrape marks across theedge of the head. These marks may be produced whenthe cartridge is worked manually through the action orfired in the firearm. As described above, these marksmay be a means to identify cartridge cases betweenscenes and other ammunition sources without a firearmbeing recovered.

Microscopic comparison of toolmarks produced byan extractor on the sides of two cartridge cases.

These toolmarks described in the preceding photo-graphs and text, especially the breech face and firingpin impressions, are routinely encountered in caseworkand are the primary areas that examiners use to deter-mine identity. However, some firearms may produce ad-ditional toolmarks on fired cases that are eitherrepresentative to a particular firearm and its function,or a group of firearms that produce atypical toolmarksdue to a particular design.


Looking down the pistol’s ejection port: A cartridge case is being pulled from the chamber of the barrel by themeans of a hook in the slide called an extractor.

WHILE THERE IS no single approach to theexamination of firearm evidence, and different labora-tory examination protocols exist, there are many thingsin common between forensic laboratories. Since a par-ticular examination is in many ways a custom productbecause of the variety of firearms evidence and investi-gation scenarios, the prosecutor must become familiarwith the general laboratory protocols utilized by theirfirearm examiner. The following is a general approachthat may be employed in an examination. It is by nomeans a standard that is used by every laboratory inevery case.

Depending on the needs of the investigation, finger-prints, trace evidence, serological stains, and other evi-dence issues may have to be resolved prior to thehandling of the firearm. For example, the more impor-tant issue in an investigation may be the fact that thevictim’s blood is in the barrel of the pistol—more im-portant than the comparison between the fatal bulletand the barrel.

Having resolved other forensic issues, the fired bul-lets and cases are examined for identifiable toolmarks.This is especially important in the evaluation of bulletsthat are damaged. If no toolmarks of value are on theevidence bullets, an identification cannot be concluded.However, for some items of evidence, certain class char-acteristics of the bullet and case may be determined.Details such as the bullet weight, bullet dimensions,composition, manufacture marks, number of lands andgrooves, direction of rifling twist, and land and grooveimpression widths may be recorded and measured. Forcartridge cases the caliber, head stamp information, caseand primer composition, shape and placement of thefiring pin impression, ejector and extractor marks,chamber marks, magazine marks and breech face im-pression pattern may be documented. This informationis then compared with a test-fired bullet and/or car-tridge cases from a firearm that may be linked to thecrime scene and/or suspect.

If more than one fired bullet and/or case is to be ex-amined, class and individual characteristics can be mi-croscopically compared to determine whether or notthe bullets or cartridge cases may be identified to eachother. This process can help determine the potentialnumber of firearms involved in the crime. If a firearmis not available, the examiner may be able to produce alist of potential firearm manufacturers that could havefired the ammunition. This list would be an investiga-tion aid, and not inclusive of all firearm sources.


THE EXAMINATION PROCESSAND TRIAL PREPARATION

Breech face and firing pin marks on two cartridgecases produced by two similar, but different pistols.The striated marks on both are due to movement ofthe barrel after firing and prior to cartridge caseejection. These marks are described as “firing pinaperture shear.”

If a firearm is submitted, the examiner may documentthe overall characteristics of the firearm, such as manu-facturer, serial number, model designation, safety func-tionality, action design, cartridge capacity, submittedammunition and/or magazines, trigger pull, and oper-ability. Once test-fired, the fired bullets and cases areexamined for class characteristics. If there are differencesin class characteristics between the firearm and evi-dence, the examination may end at this stage with anexclusion or elimination. But if the class characteristicsagree, the firearms examiner would use microscopiccomparisons between the test-fired components andthe evidence to determine if the individual detail agreessufficiently to identify the evidence bullets or cases ashaving been fired from or in the submitted firearm. Asdiscussed previously, these comparisons may also pro-duce an inconclusive result.While there is not one standard note taking or reportwriting requirement, it is generally accepted as bestpractice that the observations taken during the exami-nation are noted in the examination case file and thatany other documentation such as sketches, photographs,and reference sources are also retained. These materials

serve as a memorial of the examination and as a basis forthe determinations and conclusions. The examiner’s re-port should describe the submitted items of evidence,generally what was observed in the examination, andthe conclusions reached based on those examinations.The conclusions in the report must be supported bythe results of tests, observations, and documentation.The examination results and conclusions are typicallypeer-reviewed by another qualified examiner before thereport is released.

A crucial step in the prosecutor’s preparation for trialis a pre-trial conference with the examiner in the case.By a review of the report and the case notes, the pros-ecutor can be cognizant of what evidence was exam-ined, what examination methods were used, how theconclusions were reached, and their limits. Any addi-tional observations and conclusions not in the reportbut present in the case notes, can be learned at this stage.The prosecutor must review these documents andshould interview the witness well in advance of trial.The opposing side may, through discovery, review thesame documents and may confer with a defense expertas part of the defense trial preparation.

The pretrial conference offers numerous benefits andwill give the prosecutor a solid understanding of theitems of physical evidence, and the best order in whichto introduce them. He or she will have an understand-ing of the technical terms and will have logical, jury-friendly questions prepared for the examiner’s directtestimony. The prosecutor will know the limits of theresults and be able to anticipate answers before thequestions are asked so that he or she is prepared forcross-examination. Additionally, a pretrial conferencewill allow the prosecutor the opportunity to learn ofany potential weaknesses in the evidence, provide himor her with the opportunity to discuss possible areas ofcross-examination by the defense, and discuss testimonylikely to be offered by the defense expert (and itsstrengths and weaknesses).

The examiner must be objective and only be an ad-vocate for his or her work. The examiner must not


Measuring the width of a bullet’s land impressionusing a comparison microscope. On the left is a firedbullet, a micrometer is on the right.

weigh the testimony in favor the prosecution or de-fense. Juries are quite sensitive to any apparent fa-voritism in testimony, and the examiner will be lesscredible if this is perceived by the jury. Additionally, anyethical forensic scientist testifies to opinion only withinhis or her training and expertise. Again, pretrial confer-ences with the examiner will help in this regard.

Science and the Law: Frye/Daubert andCourt Acceptability of Firearm andToolmark Identification

It is beyond the scope of this monograph to preparethe prosecutor for all the issues that may be brought upin a Daubert or Frye admissibility hearing. Needless tosay, the prosecutor must be well prepared in advance forsuch an important court hearing. A brief discussion fol-lows, but it is incumbent upon the prosecutor to re-view and discuss with the examiner that is to betestifying, the relevant literature regarding the scientificsupport for the acceptability of “Firearm and ToolmarkIdentification,” (see Appendix, Resources).

Science is generally described as a systematically or-ganized body of knowledge about a specific subject.The word is derived from the Latin “scientia” meaning“to know.” There is a foundation of knowledge aboutfirearm and toolmark identification that has been or-ganized over time and is described in forensic text-books, scientific literature, reference material, trainingmanuals, and peer reviewed scientific journals.

The foundations of firearm identification were de-veloped using the scientific method, a process of gath-ering knowledge through observation, testing, andexperimentation. The scientific method is generally de-scribed in the following steps:

1. The problem being investigated is stated;2. Information concerning the problem is gath-

ered;3. A hypothesis is developed that may provide an

explanation for the problem under investigation;4. The hypothesis is tested by experimentation;

5. The observations and data derived by the ex-perimentation are recorded and analyzed;

6. Based on the new information, the hypothesis isdetermined to be valid or not; and

7. If the hypothesis failed, a new one is formed thatincludes the recently acquired knowledge andthe process of testing (steps four to six) is re-peated.

If the hypothesis is tested repeatedly, and has not beenfalsified or disproved, then over time the hypothesis maybe developed into a theory. The theory can then be usedby scientists to solve similar problems. The theory, how-ever, is still subjected to testing and experimentationthrough normal scientific inquiry. If it continues to re-main valid throughout this continued testing, the the-ory becomes based on an expanding body ofknowledge that is further refined to better explain thesolution of the original problem.

In Daubert, the issues that may be addressed in thedetermination of acceptability are:

• the testability of the scientific principle using the sci-entific method,

• known or potential error rate, • the existence and maintenance of standards of con-

trol, • peer review and publication, and • general acceptance in the relevant scientific commu-

nity. In this case, the relevant community is composedof practitioners in firearm and toolmark identificationscience. In preparing for a Daubert or Frye admissibility hear-

ing, keep in mind the following: • The firearms and toolmark forensic specialty is based

on the scientific method. It is an organized body ofknowledge based on a foundation and principles thatare testable by observation and design of experimentsthat seek to determine the accuracy of conclusionsmade under those principles.

• The known or potential error rate of the science is an


important consideration for the court. No human en-deavor, no matter how carefully constructed, is error-free. The court is most interested in the frequency ofmisidentification, even when using accepted tech-niques, protocols, and instrumentation. Certainly theestimation is not “0%,” which some may describe asa theoretical error rate, or that the science is infallible.While there is no known study that has determinedthe error rate in actual casework, reviews of profi-ciency testing data show that the error rate formisidentifications for firearm evidence is approxi-mately 1.0%, and for toolmark evidence it is approx-imately 1.3%.1

• It must be noted that proficiency testing was neverdesigned to determine error rate in the profession, butrather it is used as a laboratory training and quality as-surance tool. Certainly, the error rate of the individ-ual examiner may be discussed. If proficiency testsperformed by the examiner were all accurate, then theerror rate for the examiner would be 0% for thesetests. However, if an error had been made, it is criticalthat the circumstances of the error be evaluated. Per-haps, the error was made while the examiner was intraining status.

• Any potential for error is further reduced by theDaubert guideline for “the existence and maintenanceof standards of control” most commonly achieved bythe review and opinion of a second examiner. Thistype of peer review helps to ensure the accuracy ofthe results. In addition, quality control and quality as-surance measures help maintain work integrity, andare usually described in written guidelines on file inmost forensic laboratories.

• Another hallmark of a scientific discipline is the pub-lication of scientific information in peer-reviewedjournals. In this way, information on techniques andthe validity of a method is disseminated to practition-ers, who in turn may support or challenge the infor-mation. Scientific information is also disseminated viapresentations at professional association meetings andseminars.

• The relevant scientific community is represented bythe Association of Firearm and Toolmark Examiners(AFTE), an international body of practitioners in thisscience. Peer reviewed articles are published in theAFTE Journal. Additionally, standardized terms andtechnical reference information are published in theAFTE Glossary.

Automated Computer Search TechnologyAs we have seen, the firearm as a device containing

a number of separate tools can produce unique and re-producible toolmarks on fired bullets and cartridgecases. The digitizing of the surfaces of the fired cartridgecomponents in a form that can be searched in a data-base is the basis for the modern National IntegratedBallistic Information Network (NIBIN). Prior to thistechnology, the examiner had to rely on “cold search-ing” an open file of test-fired cartridges to open caseevidence. This effort was laborious, if even attempted,and was not amenable to sharing with neighboring ju-risdictions. However, in the early 1990s a prototypicalsystem to produce a digital map of the individualizingdetail on fired bullets and cartridge cases was developedand tested. This technology was developed by ForensicTechnology Incorporated (FTI) and was named the In-tegrated Ballistic Identification System (IBIS). The im-ages acquired in the crime laboratory on an IBIS wereconverted into a form so that a mathematical algorithmcould be used to compare other fired bullets and car-tridge case images in a compiled database. In this way,thousands of fired bullets and cases could be compared,scored, and images retrieved to find presumptive links toother firearm related crimes or to recovered firearms.The early testing and investigation results were so suc-cessful that the databases have been combined into anational searchable system called NIBIN.

The NIBIN Program is funded and managed by theBureau of Alcohol, Tobacco, Firearms and Explosives(ATF), but the system is run from 206 sites primarily inlocal and state crime laboratories representing 174agencies. As of 2007 approximately 1,400,000 pieces of



firearm and crime scene evidence have been enteredresulting in over 23,000 “cold hits.” NIBIN is currentlyproducing approximately 4000 cold hits a year. A coldhit is when an association is made using only this tech-nology and when no link is otherwise suspected in theinvestigation. A typical cold hit scenario could startwhen a bullet recovered at autopsy, together with casesfound at the scene, are entered into the local IBIS. Sometime later, a link is found to test-fired bullets and casesfrom a seized pistol recovered in a vehicle stop. The IBISpresents the potential hit to the examiner, and the orig-inal evidence and test fires are compared at the labora-tory to confirm the identification.

The use of both IBIS and NIBIN together could becharacterized as a search engine for firearm evidence. Apiece of evidence would be equivalent to a keyword orsubject. The keyword is searched on the Internet formore information that may be important to the reader.The closest words or terms are graded, the closestmatching information is scored the highest, and the in-

formation packets are brought up in a ranked list forfurther viewing.

This search on the Internet is similar to the bul-let/casing image search on the IBIS. An entered imageis correlated or compared to each individual image thatcorresponds to the class characteristics in the database.The images that are the most similar are scored higherthan pairs that are less similar. The complete databasecomparison results in a ranked score list. The examineris only concerned with the best scoring pairs. Thosepairs of digital images are compared visually on com-puter monitors to see which potential links should becompared microscopically. In this way, thousands of ev-idence entries can be compared not only within a lab-oratory’s database, but also within a shared database ofa number of other crime laboratory jurisdictions. TheNIBIN linking of national databases enables the exam-iner to query individual databases or groups of data-bases throughout the United States.

Place image: The IBIS instrumentation


APPENDIX

References

Drug-Linked Firearms Cases: A Primer for Prosecutors; AmericanProsecutors Research Institute, May 2005.

Firearm/toolmark Identification: Passing the Reliability Test UnderFederal and State Evidentiary Standards; Grzybowski, Miller,Moran, Murdock, and Thompson; AFTE Journal, Volume 35,Number 2, Spring 2003.

Firearm and Toolmark Identification—Meeting the Daubert Challenge;Grzybowski and Murdock; AFTE Journal, 1998: 30(1).

Firearm and Toolmark Identification Criteria: A Review of theLiterature; Nichols; Journal of Forensic Sciences, 1997 May; 42(3).

Firearm and Toolmark Identification Criteria: A Review of theLiterature, Part II; Nichols; Journal of Forensic Sciences, 2003March; 48(2).

Glossary of the Association of Firearm and toolmark Examiners;Fourth Edition.

Resources

Web-based information can be found on: www.FirearmsID.com www.AFTE.orgwww.swggun.orgwww.NIBIN.govwww.ATF.gov

For additional assistance in preparing for a FRYE/DAUBERT hearing,and answers to other gun crime prosecution questions, contact theNational District Attorneys Association’s Gun Violence ProsecutionProgram or call 703.549.9222.

Photographs and illustration credits:

Scott Doyle, Erik Dahlberg, Firearms ID.com, RobertThompson, Association of Firearm and Toolmark Examiners(AFTE), and the Bureau of Alcohol, Tobacco, Firearms andExplosives (ATF)

GLOSSARY

From the Association of Firearm and Toolmark Examiners Glossary,Fourth Edition, 2001

ActionThe working mechanism of a firearm.

Semiautomatic—A repeating firearm requiring a separate pull ofthe trigger for each shot fired, and which uses the energy ofdischarge to perform a portion of the operating or firing cycle(usually the loading portion).

Automatic—A firearm design that feeds cartridges, fires, extractsand ejects cartridge cases as long as the trigger is fully depressedand there are cartridges in the feed system. Also called “fullauto” and “machine gun.”

Revolver—A firearm, usually a handgun, with a cylinder havingseveral chambers so arranged as to rotate around an axis and bedischarged successively by the same firing mechanism. See also“revolver.”

Lever—A design wherein the breech mechanism is cycled by anexternal lever generally below the receiver.

Slide—An action that features a movable forearm which ismanually actuated in motion parallel to the barrel by the shooter.Forearm motion is transmitted to a breech bolt assembly thatperforms all the functions of the firing cycle assigned to it by thedesign. Also known as “pump action.”

Bolt—A firearm in which the breech closure: 1. is in line with the bore at all times 2. manually reciprocates to load, unload and cock, 3. is locked in place by breech bolt lugs and engages abutmentsusually in the receiver. There are two principal types of boltactions: the turn bolt and the straight pull.

Bolt FaceSee Breech Face

BoreThe interior of a barrel forward of the chamber.

Breech FaceThat part of the breechblock or breech bolt which is against thehead of the cartridge case or shotshell during firing.

APPENDIX & GLOSSARY

BulletA non-spherical projectile for use in a rifled barrel.

CartridgeA single unit of ammunition consisting of the case, primer, andpropellant with one or more projectiles. Also applies to ashotshell.

Cartridge, Center FireAny cartridge that has its primer central to the axis in the headof the case.

Cartridge, RimfireA flange-headed cartridge containing the priming mixture insidethe rim cavity.

Cartridge CaseThe container for all the other components that comprise acartridge.

ChamberThe rear part of the barrel bore that has been formed to accept aspecific cartridge. Revolver cylinders are multi-chambered.

Chamber MarksIndividual microscopic marks placed upon a cartridge case bythe chamber wall as a result of any or all of the following: (1)chambering (2) expansion during firing (3) extraction.

Class CharacteristicsMeasurable features of a specimen that indicates a restrictedgroup source. They result from design factors and are thereforedetermined prior to manufacture.

Comparison MicroscopeEssentially two microscopes connected to an optical bridge thatallows the viewer to observe two objects simultaneously with thesame degree of magnification. This instrument can have amonocular or binocular eyepiece. Sometimes referred to as a“comparison macroscope.”

Deep Hole DrillingA modern technique for barrel drilling involving rotation of theblank on a nonrotating bit, under high pressure lubrication. Also,an operation in which the depth of the hole is 10 or more timesgreater than the diameter of the drill.

EjectorA portion of a firearm’s mechanism thatejects or expelscartridges or cartridge cases from a firearm.

ExtractorA mechanism for withdrawing the cartridge or cartridge casefrom the chamber.

FirearmAn assembly of a barrel and action from which a projectile ispropelled by products of combustion.

Firing PinThat part of a firearm mechanism that strikes the primer of acartridge to initiate ignition. Sometimes called “hammer nose”or “striker.”

Function TestingThe examination of a firearm concerning its mechanicalcondition and operation. It is usually performed to determine ifall safety features are operable and/or if the firearm is capable offiring a cartridge.

GrooveSee “rifling.”

HandgunA firearm designed to be held and fired with one hand.

ImpressionContour variations on the surface of an object caused by acombination of force and motion where the motion isapproximately perpendicular to the plane being marked. Thesemarks can contain “class” and/or “individual characteristics.”

Individual Characteristics Marks produced by the random imperfections or irregularities oftool surfaces. These random imperfections or irregularities areproduced incidental to manufacture and/or caused by use,corrosion, or damage. They are unique to that tool anddistinguish it from all other tools.

LandThe raised portion between the grooves in a rifled bore.

MagazineA container for cartridges that has a spring and follower to feedthose cartridges into the chamber of a firearm. The magazinemay be detachable or an integral part of the forearm.

Obturation1. The sealing of gases due to the expansion of a cartridge case asa result of chamber pressure. 2. The sealing of gases due to the expansion and/or upset of thebullet base as it travels down the bore.

PistolA handgun in which the chamber is part of the barrel. A termsometimes used for “handgun.”

PrimerThe ignition component of a cartridge.


ProjectileAn object propelled by the force of rapidly burning gases orother means.

PropellantIn a firearm, the chemical composition, which when ignited by aprimer, generates gas. The gas propels the projectile. Also called“powder”; “gunpowder”; “smokeless powder.”

Range of Conclusions Possible When ComparingtoolmarksThe examiner is encouraged to report the objective observationsthat support the findings of toolmark examinations. Theexaminer should be conservative when reporting the significanceof these observations.

• Identification Agreement of a combination of individual characteristics andall discernible class characteristics where the extent ofagreement exceeds that which can occur in the comparison oftoolmarks made by different tools and is consistent with theagreement demonstrated by toolmarks known to have beenproduced by the same tool.

• InconclusiveA. Some agreement of individual characteristics and all

discernible class characteristics, but insufficient for anidentification.

B. Agreement of all discernible class characteristics withoutagreement or disagreement of individual characteristics dueto an absence, insufficiency, or lack of reproducibility.

C. Agreement of all discernable class characteristics anddisagreement of individual characteristics, but insufficientfor an elimination.

• EliminationSignificant disagreement of discernable class characteristicsand/or individual characteristics.

• UnsuitableUnsuitable for examination.

ReamerOne of many spiral or straight-fluted multi-edged cutting toolsused to size and shape a hole.

RevolverA firearm, usually a handgun, with a cylinder having severalchambers so arranged as to rotate around an axis and bedischarged successively by the same firing mechanism.

RifleA firearm having rifling in the bore and designed to be firedfrom the shoulder.

RiflingHelical grooves in the bore of a firearm barrel to impart rotarymotion to a projectile.

Rifling MethodsBroach, Gang—A tool having a series of cutting edges of slightlyincreasing height used to cut the spiral grooves in a barrel. Allgrooves are cut with a single pass of the broach.

Broach, Single—A non-adjustable rifling cutter which cuts all ofthe grooves simultaneously, and is used in a series of increasingdimensions until the desired groove depth is achieved.

Button—A hardened metal plug with a rifled cross sectionconfiguration. It is pushed or pulled through a drilled andreamed barrel so as to cold form the spiral grooves to the desireddepth and twist. When the carbide button was first introduced itwas described as a “swaging process” or “swaged rifling.”

Hook—A cutting tool that has a hook shape and only cuts onegroove at a time.

Scrape—A cutting tool that cuts two opposing grooves at a time.

Swage—An internal mandrel with rifling configuration thatforms rifling in the barrel by means of external hammering. Alsoknown as “hammer forging.”

RimfireSee “cartridge, rimfire.”

ShotgunA smooth bore shoulder firearm designed to fire shotshellscontaining numerous pellets or sometimes a single projectile.

Shoulder1. The act of placing a shotgun or a rifle to a shooter’s shoulder

to align the sights and fire at a target. 2.The sloping portion of a metallic cartridge case that connects

the neck and the body of a bottleneck cartridge. 3. The square or angular step between two diameters on a barrel,

pin, stud, or other part commonly used in firearms.

StriationsContour variations, generally microscopic, on the surface of anobject caused by a combination of force and motion where themotion is approximately parallel to the plane being marked.These marks can contain “class” and/or “individualcharacteristics.”

StrikerA rod-like firing pin or a separate component that impinges onthe firing pin.


Subclass CharacteristicsDiscernible surface features of an object that are more restrictivethan “class characteristics” in that they are:1. Produced incidental to manufacture.2. Are significant in that they relate to a smaller group source (a

subset of the class to which they belong).3. Can arise from a source that changes over time. Examples

would include: bunter marks, extrusion marks on pipe, etc.Caution should be exercised in distinguishing subclass

characteristics from “individual characteristics.”

Theory of Identification as itRelates to toolmarks

• The theory of identification as it pertains to the comparison oftoolmarks enables opinions of common origin to be madewhen the unique surface contours of two toolmarks are in“sufficient agreement.”

• This “sufficient agreement” is related to the significantduplication of random toolmarks as evidenced by thecorrespondence of a pattern or combination of patterns ofsurface contours. Significance is determined by thecomparative examination of two or more sets of surfacecontour patterns comprised of individual peaks, ridges andfurrows. Specifically, the relative height or depth, width,curvature and spatial relationship of the individual peaks, ridgesand furrows within one set of surface contours are defined andcompared to the corresponding features in the second set ofsurface contours. Agreement is significant when it exceeds thebest agreement demonstrated between toolmarks known tohave been produced by different tools and is consistent withagreement demonstrated by toolmarks known to have beenproduced by the same tool. The statement that “sufficientagreement” exists between two toolmarks means that theagreement is of a quantity and quality that the likelihoodanother tool could have made the mark is so remote as to beconsidered a practical impossibility.

• Currently the interpretation of individualization/identificationis subjective in nature, founded on scientific principles andbased on the examiner’s training and experience.

ToolAn object used to gain mechanical advantage. Also thought of asthe harder of two objects that, when brought into contact witheach other, results in the softer one being marked.


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Firearm Identification in the Forensic Science Laboratory