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 Justice Programs, which also includes the Bureau of Justice Statistics, the National Institute 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 represent 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 Systems

Office of Law Enforcement Standards

National 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, he

or she must be cognizant of the expectations of

today’s jury. Thanks to the modern electronic

media, use of the forensic sciences has caught

the imagination of the public, and the potential

jury pool has demonstrated that it has certain

expectations when a case is brought before it.

No matter how fantastic or erroneous these expectations are, practitioners in law enforcement

and experts in the forensic sciences have to deal

with them in a forthright manner. The best

strategy is for the prosecutor to be well acquainted with the capabilities and limits of the

forensic science disciplines that may be the

linchpin in the investigation and, more importantly, in the prosecution of a defendant at trial. 

This monograph serves to introduce the

prosecutor to the principal elements of one of

the forensic specialties, the science of “firearm

and toolmark identification.” Many of the

words and terms printed in bold in the text are

defined in the glossary. The monograph provides an introductory discussion of the specialty

of toolmark identification when the tool involved is a firearm. The tool surfaces represented here involve one or more of the

following: the interior of the barrel, the chamber, parts of the action, and ammunition magazine components. These surfaces of the firearm

can produce toolmarks on fired and unfired

ammunition components. The forensic scientist

views a “tool” as the harder of two objects

where the surface of the harder “tool” produces

toolmarks on a softer material. For example, the

tool surface of the hard barrel interior leaves

toolmarks on the softer metal of the fired bullet. Another example is when a cartridge is fired

in a firearm. The softer metal used in the cartridge case construction may show toolmarks

caused by the interior chamber and action surfaces coming in contact with the cartridge case.

The action is the firearm’s loading and firing

mechanism.

For there to be a potential for toolmark identification, the tool working surface (1) must

have individuality, and (2) the toolmarks must

be reproducible for comparisons. If it is determined that the individual character of the tool

working surface is reproduced in the toolmarks

from repetitive markings, an examiner may be

able to make an identification in later comparisons.

INTRODUCTION

Editor’s Note: Robert M. Thompson is the program manager for Forensic Data Systems in the Office of Law Enforcement Standards at the National Institute of Standards and Technology (NIST). Prior to working at NIST, Mr. Thompson was a senior firearm

and toolmark examiner for the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) Forensic Science Laboratory in Ammendale, Maryland. Mr. Thompson was an ATF examiner for 14 years, also working in the San Francisco, California ATF

Forensic Laboratory. He worked as a forensic scientist for 15 years prior to joining the ATF.

FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY 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 principles and the primary tools used in the science have

changed very little during this time. The comparison

microscope, the primary tool used by the profession,

has not changed in its basic design for almost 80 years.

Before this instrument became available, examiners relied on photomicrograph comparisons to determine

identity of fired bullets or cartridge cases, which was a

time consuming and laborious method. (The terms cartridge case, casing, and case will be interchangeable in

usage.) With the engineering of the “optical bridge,”

two compound microscopes were joined together, giving the examiner the ability to observe and compare

two objects at the same time under magnification. The

genesis of the modern comparison microscope was accelerated with the addition of microscope stages that

were designed for the mounting of fired bullets, cases,

and other items bearing toolmarks. The science of

firearm identification was soon propelled forward in

forensic investigations in this nation and worldwide.

Today, firearm units in crime laboratories might use

other complimentary microscopic and photographic

instrumentation, but for matters concerning the identification of toolmarks on fired bullets, cases or any

other object, the comparison microscope is an absolutely necessary instrument. 

The recent computer technology for searching image

databases for presumptive linkages is demonstrated by

the installation and use of the National Integrated Ballistic Information Network (NIBIN). NIBIN has a firm

foundation of acceptance in the forensic science community and the courts. 

As was described previously, the forensic science of

firearm identification is a specialized sub-specialty of

toolmark identification specifically related to the

firearm mechanism’s working surfaces. The firearm is

made up of a number of tools, many of which come

into contact with, and leave toolmarks on the softer

metal of the cartridge case and/or bullet. The firearm,

as with any other tool, has features that were designed

by the factory. Features that are determined by the manufacturer include the size of the cartridge chambered

by the firearm, the orientation of the extractor and

ejector, and the number, width and twist direction of

the land and grooves of the barrel rifling. These characteristics can be imparted as toolmarks on the fired

bullet and case during firing, and can be classified by

their class characteristics. These class characteristics are

typically the first classification of toolmark evidence that

the examiner seeks in the examination. Class characteristics help narrow the population of potential firearm

sources. The following experience common to us all

gives an example of sorting using class characteristics:

You are leaving a store and have to find your

car in a large, crowded parking lot. You begin by

looking for a certain vehicle type (SUV, convertible, sedan, etc.), make (Ford, Chevy, Volvo, etc),

A comparison microscope

8 FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY

THE SCIENCE OF

FIREARM IDENTIFICATION
model, and color. You are looking for the class

characteristics of your car.

If the class characteristics agree in every respect with

the evidence item (i.e., the cartridge case or the recovered bullet) and with the test-fires from a suspect

firearm, the examiner then uses the comparison microscope to compare the individual characteristics of

both evidence and test toolmarks. Individual characteristics are random in nature, usually arising from the tool

working surface incidental to manufacture, but can also

be the result of use, wear, and possible care and/or abuse

of the tool.

Building on the example of finding your car

in the parking lot, you find what appears to be

your car, but you know it is a popular model and

have seen very similar cars in other parking lots

and on the road. So you approach the car that

looks like yours and you search for those individual characteristics that make it your own. For example, you look at the license plate, window

decals, dings and dents. These all together confirm

that the car is yours and not someone else’s. You do

not search for every individual feature that you

know is on your vehicle, but just enough to determine its identity.

The characteristics that make the tool surface unique

are called individual characteristics. When these characteristics are compared in toolmarks, and sufficient

agreement is found, an identification can be established.

These characteristics are from imperfections on the tool

surface that make the toolmark. The imperfections, typically microscopic, usually arise during the tool manufacturing process. In addition, the surface may also gain

imperfections and irregularities through use, wear, corrosion, and damage. Remember your car in the parking lot? Individual characteristics would be the dings

and dents, the license tag, the rust spots, and the windshield crack. As you can see, these characteristics would

be acquired over time compared to the few that would

be seen on a new show room car.

There is a toolmark classification termed subclass

characteristics, sometimes referred to as “carryover.”

These tool surface characteristics are incidental to manufacture, are significant in that they relate to a subgroup

from which they belong, and arise from a tool source

that can change over time. Subclass characteristics can

be reproduced on a limited number of tools. Therefore,

the examiner cannot base identification on toolmarks

derived from such a source knowing there is a good

chance that such a toolmark could originate from several firearm barrels. 

A source of such a subclass characteristic may be produced during the cutting of barrel grooves in rifled barrels, if, for example, during the cutting of many barrels

on an assembly line, one of the cutters develops a large

chip that is not noticed by the machinist or quality control experts. The chip on the cutter may produce a

coarse imperfection in an otherwise cleanly cut groove.

The detail from this defect may be reproduced on a

number of consecutive barrels (i.e. carried over), until

the cutting tool is discarded or re-sharpened. The experienced firearm examiner is aware of such artifacts

occurring in the barrel forming process, and understands that these types of coarse, continuous toolmarks,

while useful in the examination and comparison

process, cannot be a basis for an identification. This is

one of a number of instances in firearm and toolmark

comparisons in which subclass characteristics have to

be considered before an identification of a toolmark

source is concluded.

Toolmarks generally appear in two forms: striated

and impressed. Striated toolmarks are formed when a

tool-working surface is placed on another surface and

moved parallel to that surface. In other words, a tool

makes a scratch or scrape mark on the surface of anFIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY 9
other object. The detail in this toolmark has the appearance of parallel lines, called stria. Under the microscope the stria are seen as a profile consisting of hills,

valleys and ridges. If the stria is very shallow, the toolmark appears as a pattern of lines. Impressed toolmarks

are formed when the tool surface is forced perpendicularly to another surface. This toolmark has the appearance of having been stamped. Due to the process of

impressing a toolmark, stria production is very limited,

and may not be formed at all. Instead, the tool working

surface imperfections give the negative detail in the

toolmark. The examiner uses a comparison microscope

to determine identification for both striated toolmarks

and impressed toolmarks. 

An example of a striated toolmark is the action

that occurs with a car’s windshield wiper against a

wet windshield. If the wiper is well worn, nicks of

various sizes will be randomly present on the

blade. When the wiper is used on the windshield,

a pattern of lines is drawn across the arc of the

wiper movement. The placement of these imperfections cannot be accidentally duplicated on any

other blade length, and the pattern of stria is individual to that particular blade. Additionally, the

toolmark in the windshield is duplicated on each

stroke, exemplifying the reproducibility of the

toolmark. 

Similarly, the impressed toolmark can be

characterized by the stamping of coins. The tool

that impressed the coin had the negative profile of

the coin. The coin produced has the impression of

the tool on its softer metal. The class characteristics of a 2004 dime are visually apparent. However,

microscopic imperfections impressed on the dime

may be used to identify which specific tool in the

mint was used to produce the coin.

For the science of toolmark identification, the underlying hypothesis is that a toolmark can be identified

to a specific tool that produced it, to the practical exclusion of all other tools. Clearly, it is impossible to prove

this hypothesis by testing all tools ever produced in the

world. Instead, identification must be inferred, based on

observation and experimentation. Over many years scientists have documented that the surfaces of tools that

make toolmarks are microscopically dissimilar and individual in nature. This dissimilarity is observed and potentially quantifiable in what is called “known

non-match” comparisons. While there is a potential for

random agreement to a small extent, this agreement

does not reach the quality and quantity shown between

toolmarks made by the same tool working surface, or

“known matches.” Therefore, if the agreement of toolmarks is of sufficient quality and quantity that is expected from one tool, and greater in quality and

quantity than has been demonstrated by the best

“known non-match” toolmarks from different tools, an

identification can be made between the two toolmarks.

However, as stated before, prior to the determination

of identification, the influence of sub-class characteristics has to be eliminated. The human being is very cognitive of the environment, and one of the hallmarks of

human reason is the detection of patterns, whether by

the senses, or by circumstances in time.

Consider your drive to and from work. Even in

busy traffic, you as an experienced driver, tune out

many of the circumstances of a routine commute.

Each drive is different and has random circumstances that vary to a degree over the weeks of the

same commute. One day you see a new sports car

that catches your eye because you appreciate sports

cars. As it passes by you note its color, its lines, and

perhaps 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 new

sports car pass you on the road. Now, you are really interested, because this rare event just happened in one trip in a few minutes. You carefully

compare car number two with the mental notes

you made of car number one. You know that this

1 0 FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY
may be coincidence, the cars may not be produced

with many options, and it may be rare to see two

cars in such close proximity. It is an interesting coincidence (a non-match). Consider what you start

to realize when you pass another and another—a

number of similar cars. You now know that something special might be happening based on the coincidental discovery of these cars in such a close

space of time. Something else must be going on

to make this a singular event. Perhaps there was a

car club, a manufacturer test market, or an auto

show. Any idea that this event happened simply by

chance is quickly dismissed, with confidence (a

match). 

The largest organization that supports the interchange

of scientific information concerning firearm and toolmark science is the Association of Firearm and Toolmark Examiners (AFTE), which publishes the AFTE

Journal. The AFTE Journal is peer-reviewed by an editorial committee, with a section in each issue set aside

for responses by the readers. Peer review helps ensure

that open discussion among practitioners is maintained

and that any information being disseminated is accurate and reliable.

In 1992, AFTE adopted the “Theory of Identification”

which reads:

1. The theory of identification as it pertains to the

comparison of toolmarks enables opinions of

common origin to be made when the unique

surface contours of two toolmarks are in “sufficient agreement.”

2. This “sufficient agreement” is related to the significant duplication of random toolmarks as evidenced by a pattern or combination of patterns

of surface contours. Significance is determined

by the comparative examination of two or more

sets of surface contour patterns comprised of individual peaks, ridges and furrows. Specifically,

the relative height or depth, width, curvature

and spatial relationship of the individual peaks,

ridges and furrows within one set of surface

contours are defined and compared to the corresponding features in the second set of surface

contours. Agreement is significant when it exceeds the best agreement demonstrated between

toolmarks known to have been produced by different tools and is consistent with agreement

demonstrated by toolmarks known to have been

produced by the same tool. The statement that

“sufficient agreement” exists between two toolmarks means that the agreement is of a quantity

and quality that the likelihood of another tool

making the mark is so remote as to be considered a practical impossibility.

3. Currently the interpretation of individualization/identification is subjective in nature,

founded on scientific principles and based on

the examiner’s training and experience.

As part of the standardization of terms and conclusions

for the firearms examiner to employ, AFTE developed

a range of conclusions based on the Theory of Identification. The examiner would conservatively describe

objective observations and the results of examinations,

as follows:

Identification: 

Agreement of a combination of individual characteristics and all discernible class characteristics where the

extent of agreement exceeds that which can occur in

the comparison of toolmarks made by different tools

and is consistent with the agreement demonstrated by

toolmarks known to have been produced by the same

tool.

Inconclusive:

a. Some agreement of individual characteristics and all

discernible class characteristics, but insufficient for an

identification.

FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY 1 1
b. Agreement of all discernible class characteristics

without agreement or disagreement of individual

characteristics due to an absence, insufficiency, or lack

of reproducibility.

c. Agreement of all discernible class characteristics and

disagreement of individual characteristics, but insufficient for an elimination.

Elimination:

Significant disagreement of discernible class characteristics and/or individual characteristics.

Unsuitable:

Unsuitable for examination.

The Production of Firearm

Toolmarks on the Fired Cartridge

When toolmarks are made on the fired bullet and

cartridge case, their general appearance and orientation

originate from the class characteristics of the firearm

producing those marks. Routinely, when a firearm is

not collected as part of an investigation, the firearm examiner measures and characterizes the marks (both striated and impressed) found on the bullet and/or

cartridge case. Then the examiner compares the observations and data to reference literature and databases,

and produces a list of possible firearm manufacturers

and possibly models, that could be the source of the evidence. While such a list is not all inclusive of all possible manufacturers, it may be an aid in an investigation

where a suspect firearm was not recovered. However, if

a suspect firearm is recovered, the firearms examiner

will determine if the firearm has the correct class characteristics by examination and test firing, and then if

the class characteristics agree, will microscopically compare the test-fired bullets and cartridge cases to the exhibits collected in the investigation. 

To better understand the placement of toolmarks on

fired cartridge components, an understanding of firearm

types, actions, ammunition, and firearm toolmark producing surfaces is necessary.

Firearm Types

The basic types of firearms are handguns and shoulder

arms. Handguns are designed to be fired by one hand

without support from the body. A shoulder arm is designed with a stock to be fired while being supported

by the shoulder.

• Handguns

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

1 2 FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY

Semiautomatic 9mm pistol

Double-action 44 Magnum Revolver with Scope
Revolver—A firearm that has a number of chambers in

a cylinder that rotates on an axis; during successive firing, a chamber rotates and aligns with the barrel.

• Shoulder Arms

Rifle—A firearm that has a rifled barrel and is designed

to be fired from the shoulder

Shotgun—A smooth bore barreled shoulder firearm designed to fire shotshells that contain numerous pellets,

or a single projectile.

Modern Firearm Actions

Firearms have loading and firing mechanisms called

actions. Modern firearms may have differing actions

depending on the design of the firearm. The most common forms are semiautomatic, automatic (also known as

full auto or machine gun), revolver, lever, slide (or

pump), and bolt actions. A “firing cycle” is composed

of the actions performed by the shooter and the firearm

mechanism to fire a cartridge, with the subsequent

readying of the firearm for a discharge of the next cartridge. The most commonly encountered firearm actions are:

• Semiautomatic—A firearm that requires a separate pull

of the trigger for each shot, and uses energy from the

discharge to perform a portion of the operation or

firing cycle, usually the extraction and loading portions.

• Automatic—A firearm that feeds cartridges, fires, extracts and ejects cartridge cases continuously for as

long as the trigger is fully depressed and there are cartridges in the feed system.

• Revolver—A firearm that has a number of chambers in

a cylinder that rotates on an axis; during successive firing, a chamber rotates and aligns with the barrel.

• Lever—A firearm wherein the breech mechanism is

cycled by an external lever generally below the receiver.

• Slide (pump)—A firearm with a movable forearm that

is moved in line with the barrel by the shooter. This

motion is connected to the breech bolt assembly,

which performs the functions of the firing cycle that

is assigned to it.

• Bolt—A firearm where the breech closure is in line

with the barrel; the closure manually reciprocates to

load, unload, and cock; and locks in place by breech

bolt 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 and

movies. For example, a suspect was arrested with “bullets” in his pocket, a semiautomatic rifle that can carry

many “bullets,” or a cowboy in a shootout is “out of

bullets.” However, the unit of ammunition is properly

termed a cartridge. The cartridge consists of a case, a

primer, propellant (powder), and one or more projectiles. The projectile is the true bullet. (In some areas of

FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY 1 3

Lever-action Rifle

Pump-action shotgun
the United States the bullet may also be termed “pellet.”) Experienced shooters or otherwise informed jurors will be very aware of the distinction between

cartridge and bullet.

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

metallic cartridge are rim fire and center fire. Rim fire

cartridges, common with .22 calibers have the primer

compound inside the rim of the case head. The primer

compound is shock sensitive and emits a hot jet of flame

onto the powder when the case rim is struck by the firing pin of the firearm, similar to a toy cap being struck.

Center fire cartridges have a separate primer seated in

the center portion of the case head. When the primer

is struck, the jet of flame passes from the primer through

an internal opening in the bottom of the case called the

flash hole, thereby igniting the powder.

1 4 FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY

Center Fire Cartridges

Center Fire Cartridge

22 Caliber Rimfire

Revolver Cartridge Lead Hollow Point Bullet
Toolmarks on Fired Ammunition

Components and Their Sources

Fired Bullets

Fired bullets have impressed and striated toolmarks

that are generated by the tool working surface of the

rifled bore of the barrel. Rifling is the construction of

helical grooves in the bore that impart a rotary motion

or spin to a fired bullet, thereby giving the bullet more

range, stability, and accuracy. When the powder in the

cartridge starts burning after ignition, the extreme pressure produced by the gasses causes the rear of the bullet to deform slightly and swell to fill the inside of the

barrel. The bullet deformation helps seal the gasses behind it as it travels down the barrel. This bullet deformation effect is called obturation. The sides of the

bullet are engaged by the rifling, and the soft metal is

impressed by the raised portion of the rifling called

lands and alternately, may fill in the rifling between the

lands called grooves. As the bullet travels down the barrel, the soft metal on the sides are engraved by the rifling until it leaves the barrel. Some of the class

characteristics found on a fired bullet are (1) the caliber

of the bullet (diameter), (2) the number of lands and

grooves, (3) the twist of the rifling (left or right), and (4)

the widths of the land and groove impressions. The ability to determine all or some of a fired bullet’s class characteristics may be limited due to the condition of the

bullet when it was recovered.

FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY 1 5

A fired cartridge case and a fired bullet. Note the

firing pin impression on the primer of the cartridge case and the land and groove engraving

on the bullet.

A fired bullet with barrel rifling impressions on

the sides.

A bullet jacket, typically found in casework.
Barrel Manufacture Methods and the Basis

for Identification of Fired Bullets

There are, and have been, numerous manufacturers of

firearms with rifled barrels. Many manufacturers make

rifled barrels as their main product. Each manufacturer

produces rifled barrels in a particular manner best suited

to the company’s needs. However, they all use basic production methods to manufacture rifled barrels.

Briefly, the basic steps to make a rifle barrel from a

length of steel bar stock are:

• The barrel is drilled lengthwise with a tool called a

deep hole gun drill. This produces a hole, which at

this stage, is not adequately smooth or sized to the

specified dimension of the designed final bore size.

• A cutting tool called a “reamer” finishes the bore by

removing coarse material from the hole drilling

process, and perfects a true circular hole. After this action, the bore is now the proper size, relatively

smooth, and consistent dimensionally down the

length of the bore. The reamer leaves fine annular ring

toolmarks that are close to perpendicular to the bore

axis.

• The rifling in the bore may be produced by one of

the following methods:

• A cutting tool that cuts grooves singularly, or

as a “gang” where multiple grooves are cut to

the desired depth. The grooves cut one at a

time are made by a hook cutter, two at a time

with a scrape cutter. Multiple grooves cut in

one pass are made by a gang broach. Of these

methods, the gang broach is commonly used

today for cut rifling.

• Button rifling (also termed button swage): In

this method a very hard tungsten button

which has the reverse cross section of the desired rifling is pushed or pulled through a

bore that has a smaller diameter than the button. Under high pressure, the metal flows

around the button surface as it passes down

the barrel. The rifling is “ironed in” to the

barrel interior and no metal is removed.

• Hammer forging: (In some ways may be

imagined as the reverse of swaging.) A mandrel with the cross section of the rifled bore

interior is placed in a slightly larger barrel

bore. A system of large hammers, under

tremendous force, pound from the outside of

the barrel onto the mandrel inside, much like

a blacksmith hammers a red-hot horseshoe

into shape. The finished bore will have the

imprint of the mandrel’s rifling impressed on

the interior.

1 6 FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY

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

Broach Cut 6-Left
Each of these rifling processes has a number of important steps. The tools cutting and forming the rifling

undergo change as the products are formed in the manufacturing process and wear down during the lifetime

of the tool. If a tool becomes too dull, or does not perform to tolerance, then it must be sharpened, reconditioned, or replaced. At the microscopic level, the tool

working surface—the barrel—has its own individuality. That individuality can be reproduced in the engraved toolmarks on the fired bullet. Based on this

individuality of the interior of the barrel, bullets can be

identified to a particular barrel.

FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY 1 7

Barrel rifling

A button swage used to form rifling in a barrel.

A gang broach that progressively cuts rifling in

a barrel.

Cross section of a barrel showing toolmarks

that survive the rifling procedure.
Fired Cartridge Cases

Breech/bolt Face and Firing Pin Toolmark Individuality on Fired Cartridge Cases

Fired cartridge cases are often left at shooting scenes

because the shooters are not inclined to waste time

searching for the ejected and fired cartridge cases. A

fired case may have a number of surfaces that bear both

impressed and striated toolmarks from the firearm

mechanism that fired it. As with bullets, cartridge cases

can also bear class characteristics of the firearm that may

provide the examiner with information needed to assemble a list of firearm manufacturers in the event the

firearm itself is unavailable for comparison.

1 8 FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY

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 cartridge

primer, it leaves an impressed toolmark on the soft

metal of the primer, and any microscopic imperfections

on the surface of the firing pin can be transferred onto

the primer. These toolmarks are usually individual in

nature and can be reproduced during firings. 

FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY 1 9

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 in

the upper left, 9 to 12 o’clock.
As the powder burns and creates pressure, the case swells

inside the chamber and seals the gasses from escaping,

except down the barrel behind the bullet. This sealing

effect, as described with fired bullets, is also called obturation. The softer metal of the case (brass, aluminum,

soft steel) may receive toolmark impressions from the

chamber sides called chamber marks. As the bullet

passes down and out of the barrel, the head of the case

impacts the breech or bolt face that holds the case in the

chamber. The imperfections of the breech face impress

a negative impression on the case head and are called

breech face marks.

2 0 FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY

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

Microscopic comparison of chamber marks on the

sides of two cartridge cases.
FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY 2 1

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 process

clears the chamber area for subsequent loading of another cartridge into the chamber. The ejector is typically attached to the frame, remains stationary, and kicks

the case out of the ejection port after chamber extraction. If there is enough force in this event, the case will

have an impression of the ejector, and this toolmark

may be identifiable to a particular ejector. (Not all ejectors are of this design. Some are integral to the bolt or

firing pin.

Action Marks on Fired Cartridge Cases and

Unfired Cartridges; Investigation Potentials

A number of firearm tool surfaces may leave marks

on the cartridge case when a cartridge is fired in a

firearm. Toolmarks can be produced when a cartridge is

loaded, chambered, and extracted without a discharge.

Take for example a semiautomatic pistol. The ammunition magazine may leave toolmarks on the side of the

cases when the cartridges come in contact with the

magazine lips. The cartridges in the magazine are under

spring tension and are held in place by magazine lips.

The lips may scrape the sides of each case as they are

pushed into a chamber, or as they are loaded into, or

removed from, the magazine by hand. These toolmarks

on the cases may be produced while the magazine is

unattached to the firearm. If there is sufficient individualizing detail in these marks (which can be very limited), an identification to a particular magazine may be

established. This is important to an investigator because

a magazine left at the scene, or confiscated from a suspect, may be compared to ammunition or fired cases

recovered at the scene, or ammunition that is seized in

the course of the investigation, even when the firearm

is not recovered.

2 2 FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY

Microscopic comparison of ejector impressions on

two cartridge cases.

A cartridge in an ammunition magazine. The top

cartridge held in place by the two magazine lips.
FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY 2 3

The side view of an ammunition magazine with the

orientation 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 an

ammunition magazine.
2 4 FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY

A view through the ejection port of a pistol. To the left is the pistol slide and breech face. In the center of the

breech face is the firing pin aperture. To the left of the breech face is the extractor. To the lower right of the

breech face is the ejector. Both magazine lips can be seen below and forward of the slide.

Similarly, cartridges may be loaded into, and extracted from, a firearm chamber without firing. A tool

that helps this process is called an extractor, which is

found on the bolt or slide of the firearm. The tool resembles a claw, which grabs the case head at the base of

the cartridge, and may produce scrape marks across the

edge of the head. These marks may be produced when

the cartridge is worked manually through the action or

fired in the firearm. As described above, these marks

may be a means to identify cartridge cases between

scenes and other ammunition sources without a firearm

being recovered.

Microscopic comparison of toolmarks produced by

an extractor on the sides of two cartridge cases.
These toolmarks described in the preceding photographs and text, especially the breech face and firing

pin impressions, are routinely encountered in casework

and are the primary areas that examiners use to determine identity. However, some firearms may produce additional toolmarks on fired cases that are either

representative to a particular firearm and its function,

or a group of firearms that produce atypical toolmarks

due to a particular design.

FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY 2 5

Looking down the pistol’s ejection port: A cartridge case is being pulled from the chamber of the barrel by the

means of a hook in the slide called an extractor.
WHILE THERE IS no single approach to the

examination of firearm evidence, and different laboratory examination protocols exist, there are many things

in common between forensic laboratories. Since a particular examination is in many ways a custom product

because of the variety of firearms evidence and investigation scenarios, the prosecutor must become familiar

with the general laboratory protocols utilized by their

firearm examiner. The following is a general approach

that may be employed in an examination. It is by no

means a standard that is used by every laboratory in

every case. 

Depending on the needs of the investigation, fingerprints, trace evidence, serological stains, and other evidence issues may have to be resolved prior to the

handling of the firearm. For example, the more important issue in an investigation may be the fact that the

victim’s blood is in the barrel of the pistol—more important than the comparison between the fatal bullet

and the barrel. 

Having resolved other forensic issues, the fired bullets and cases are examined for identifiable toolmarks.

This is especially important in the evaluation of bullets

that are damaged. If no toolmarks of value are on the

evidence bullets, an identification cannot be concluded.

However, for some items of evidence, certain class characteristics of the bullet and case may be determined.

Details such as the bullet weight, bullet dimensions,

composition, manufacture marks, number of lands and

grooves, direction of rifling twist, and land and groove

impression widths may be recorded and measured. For

cartridge cases the caliber, head stamp information, case

and primer composition, shape and placement of the

firing pin impression, ejector and extractor marks,

chamber marks, magazine marks and breech face impression pattern may be documented. This information

is then compared with a test-fired bullet and/or cartridge cases from a firearm that may be linked to the

crime scene and/or suspect.

If more than one fired bullet and/or case is to be examined, class and individual characteristics can be microscopically compared to determine whether or not

the bullets or cartridge cases may be identified to each

other. This process can help determine the potential

number of firearms involved in the crime. If a firearm

is not available, the examiner may be able to produce a

list of potential firearm manufacturers that could have

fired the ammunition. This list would be an investigation aid, and not inclusive of all firearm sources.

2 6 FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY

THE EXAMINATION PROCESS

AND TRIAL PREPARATION

Breech face and firing pin marks on two cartridge

cases produced by two similar, but different pistols.

The striated marks on both are due to movement of

the barrel after firing and prior to cartridge case

ejection. These marks are described as “firing pin

aperture shear.”
If a firearm is submitted, the examiner may document

the overall characteristics of the firearm, such as manufacturer, serial number, model designation, safety functionality, action design, cartridge capacity, submitted

ammunition and/or magazines, trigger pull, and operability. Once test-fired, the fired bullets and cases are

examined for class characteristics. If there are differences

in class characteristics between the firearm and evidence, the examination may end at this stage with an

exclusion or elimination. But if the class characteristics

agree, the firearms examiner would use microscopic

comparisons between the test-fired components and

the evidence to determine if the individual detail agrees

sufficiently to identify the evidence bullets or cases as

having been fired from or in the submitted firearm. As

discussed previously, these comparisons may also produce an inconclusive result.

While there is not one standard note taking or report

writing requirement, it is generally accepted as best

practice that the observations taken during the examination are noted in the examination case file and that

any 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 for

the determinations and conclusions. The examiner’s report should describe the submitted items of evidence,

generally what was observed in the examination, and

the conclusions reached based on those examinations.

The conclusions in the report must be supported by

the results of tests, observations, and documentation.

The examination results and conclusions are typically

peer-reviewed by another qualified examiner before the

report is released. 

A crucial step in the prosecutor’s preparation for trial

is a pre-trial conference with the examiner in the case.

By a review of the report and the case notes, the prosecutor can be cognizant of what evidence was examined, what examination methods were used, how the

conclusions were reached, and their limits. Any additional observations and conclusions not in the report

but present in the case notes, can be learned at this stage.

The prosecutor must review these documents and

should interview the witness well in advance of trial.

The opposing side may, through discovery, review the

same documents and may confer with a defense expert

as part of the defense trial preparation. 

The pretrial conference offers numerous benefits and

will give the prosecutor a solid understanding of the

items of physical evidence, and the best order in which

to introduce them. He or she will have an understanding of the technical terms and will have logical, juryfriendly questions prepared for the examiner’s direct

testimony. The prosecutor will know the limits of the

results and be able to anticipate answers before the

questions are asked so that he or she is prepared for

cross-examination. Additionally, a pretrial conference

will allow the prosecutor the opportunity to learn of

any potential weaknesses in the evidence, provide him

or her with the opportunity to discuss possible areas of

cross-examination by the defense, and discuss testimony

likely to be offered by the defense expert (and its

strengths and weaknesses).

The examiner must be objective and only be an advocate for his or her work. The examiner must not

FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY 2 7

Measuring the width of a bullet’s land impression

using a comparison microscope. On the left is a fired

bullet, a micrometer is on the right.
weigh the testimony in favor the prosecution or defense. Juries are quite sensitive to any apparent favoritism in testimony, and the examiner will be less

credible if this is perceived by the jury. Additionally, any

ethical forensic scientist testifies to opinion only within

his or her training and expertise. Again, pretrial conferences with the examiner will help in this regard. 

Science and the Law: Frye/Daubert and

Court Acceptability of Firearm and

Toolmark Identification

It is beyond the scope of this monograph to prepare

the prosecutor for all the issues that may be brought up

in a Daubert or Frye admissibility hearing. Needless to

say, the prosecutor must be well prepared in advance for

such an important court hearing. A brief discussion follows, but it is incumbent upon the prosecutor to review and discuss with the examiner that is to be

testifying, the relevant literature regarding the scientific

support for the acceptability of “Firearm and Toolmark

Identification,” (see Appendix, Resources).

Science is generally described as a systematically organized body of knowledge about a specific subject.

The word is derived from the Latin “scientia” meaning

“to know.” There is a foundation of knowledge about

firearm and toolmark identification that has been organized over time and is described in forensic textbooks, scientific literature, reference material, training

manuals, and peer reviewed scientific journals. 

The foundations of firearm identification were developed using the scientific method, a process of gathering knowledge through observation, testing, and

experimentation. The scientific method is generally described in the following steps:

1. The problem being investigated is stated;

2. Information concerning the problem is gathered;

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 experimentation are recorded and analyzed;

6. Based on the new information, the hypothesis is

determined to be valid or not; and

7. If the hypothesis failed, a new one is formed that

includes the recently acquired knowledge and

the process of testing (steps four to six) is repeated.

If the hypothesis is tested repeatedly, and has not been

falsified or disproved, then over time the hypothesis may

be developed into a theory. The theory can then be used

by scientists to solve similar problems. The theory, however, is still subjected to testing and experimentation

through normal scientific inquiry. If it continues to remain valid throughout this continued testing, the theory becomes based on an expanding body of

knowledge that is further refined to better explain the

solution of the original problem. 

In Daubert, the issues that may be addressed in the

determination of acceptability are:

• the testability of the scientific principle using the scientific method, 

• known or potential error rate, 

• the existence and maintenance of standards of control, 

• peer review and publication, and 

• general acceptance in the relevant scientific community. In this case, the relevant community is composed

of practitioners in firearm and toolmark identification

science. 

In preparing for a Daubert or Frye admissibility hearing, keep in mind the following: 

• The firearms and toolmark forensic specialty is based

on the scientific method. It is an organized body of

knowledge based on a foundation and principles that

are testable by observation and design of experiments

that seek to determine the accuracy of conclusions

made under those principles.

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

2 8 FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY
important consideration for the court. No human endeavor, no matter how carefully constructed, is errorfree. The court is most interested in the frequency of

misidentification, even when using accepted techniques, protocols, and instrumentation. Certainly the

estimation is not “0%,” which some may describe as

a theoretical error rate, or that the science is infallible.

While there is no known study that has determined

the error rate in actual casework, reviews of proficiency testing data show that the error rate for

misidentifications for firearm evidence is approximately 1.0%, and for toolmark evidence it is approximately 1.3%.1

• It must be noted that proficiency testing was never

designed to determine error rate in the profession, but

rather it is used as a laboratory training and quality assurance tool. Certainly, the error rate of the individual examiner may be discussed. If proficiency tests

performed by the examiner were all accurate, then the

error rate for the examiner would be 0% for these

tests. However, if an error had been made, it is critical

that the circumstances of the error be evaluated. Perhaps, the error was made while the examiner was in

training status. 

• Any potential for error is further reduced by the

Daubert guideline for “the existence and maintenance

of standards of control” most commonly achieved by

the review and opinion of a second examiner. This

type of peer review helps to ensure the accuracy of

the results. In addition, quality control and quality assurance measures help maintain work integrity, and

are usually described in written guidelines on file in

most forensic laboratories.

• Another hallmark of a scientific discipline is the publication of scientific information in peer-reviewed

journals. In this way, information on techniques and

the validity of a method is disseminated to practitioners, who in turn may support or challenge the information. Scientific information is also disseminated via

presentations at professional association meetings and

seminars. 

• The relevant scientific community is represented by

the Association of Firearm and Toolmark Examiners

(AFTE), an international body of practitioners in this

science. Peer reviewed articles are published in the

AFTE Journal. Additionally, standardized terms and

technical reference information are published in the

AFTE Glossary. 

Automated Computer Search Technology

As we have seen, the firearm as a device containing

a number of separate tools can produce unique and reproducible toolmarks on fired bullets and cartridge

cases. The digitizing of the surfaces of the fired cartridge

components in a form that can be searched in a database is the basis for the modern National Integrated

Ballistic Information Network (NIBIN). Prior to this

technology, the examiner had to rely on “cold searching” an open file of test-fired cartridges to open case

evidence. This effort was laborious, if even attempted,

and was not amenable to sharing with neighboring jurisdictions. However, in the early 1990s a prototypical

system to produce a digital map of the individualizing

detail on fired bullets and cartridge cases was developed

and tested. This technology was developed by Forensic

Technology Incorporated (FTI) and was named the Integrated Ballistic Identification System (IBIS). The images acquired in the crime laboratory on an IBIS were

converted into a form so that a mathematical algorithm

could be used to compare other fired bullets and cartridge 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 to

other firearm related crimes or to recovered firearms.

The early testing and investigation results were so successful that the databases have been combined into a

national searchable system called NIBIN.

The NIBIN Program is funded and managed by the

Bureau of Alcohol, Tobacco, Firearms and Explosives

(ATF), but the system is run from 206 sites primarily in

local and state crime laboratories representing 174

agencies. As of 2007 approximately 1,400,000 pieces of

FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY 2 9
3 0 FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY

firearm and crime scene evidence have been entered

resulting in over 23,000 “cold hits.” NIBIN is currently

producing approximately 4000 cold hits a year. A cold

hit is when an association is made using only this technology and when no link is otherwise suspected in the

investigation. A typical cold hit scenario could start

when a bullet recovered at autopsy, together with cases

found at the scene, are entered into the local IBIS. Some

time later, a link is found to test-fired bullets and cases

from a seized pistol recovered in a vehicle stop. The IBIS

presents the potential hit to the examiner, and the original evidence and test fires are compared at the laboratory to confirm the identification.

The use of both IBIS and NIBIN together could be

characterized as a search engine for firearm evidence. A

piece of evidence would be equivalent to a keyword or

subject. The keyword is searched on the Internet for

more information that may be important to the reader.

The closest words or terms are graded, the closest

matching information is scored the highest, and the information packets are brought up in a ranked list for

further viewing. 

This search on the Internet is similar to the bullet/casing image search on the IBIS. An entered image

is correlated or compared to each individual image that

corresponds to the class characteristics in the database.

The images that are the most similar are scored higher

than pairs that are less similar. The complete database

comparison results in a ranked score list. The examiner

is only concerned with the best scoring pairs. Those

pairs of digital images are compared visually on computer monitors to see which potential links should be

compared microscopically. In this way, thousands of evidence entries can be compared not only within a laboratory’s database, but also within a shared database of

a number of other crime laboratory jurisdictions. The

NIBIN linking of national databases enables the examiner to query individual databases or groups of databases throughout the United States.

Place image: The IBIS instrumentation
FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY 3 1

APPENDIX

References

Drug-Linked Firearms Cases: A Primer for Prosecutors; American

Prosecutors Research Institute, May 2005.

Firearm/toolmark Identification: Passing the Reliability Test Under

Federal 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 the

Literature; Nichols; Journal of Forensic Sciences, 1997 May; 42(3).

Firearm and Toolmark Identification Criteria: A Review of the

Literature, Part II; Nichols; Journal of Forensic Sciences, 2003

March; 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.org

www.swggun.org

www.NIBIN.gov

www.ATF.gov

For additional assistance in preparing for a FRYE/DAUBERT hearing,

and answers to other gun crime prosecution questions, contact the

National District Attorneys Association’s Gun Violence Prosecution

Program or call 703.549.9222.

Photographs and illustration credits:

Scott Doyle, Erik Dahlberg, Firearms ID.com, Robert

Thompson, Association of Firearm and Toolmark Examiners

(AFTE), and the Bureau of Alcohol, Tobacco, Firearms and

Explosives (ATF)

GLOSSARY

From the Association of Firearm and Toolmark Examiners Glossary,

Fourth Edition, 2001

Action

The working mechanism of a firearm. 

Semiautomatic—A repeating firearm requiring a separate pull of

the trigger for each shot fired, and which uses the energy of

discharge to perform a portion of the operating or firing cycle

(usually the loading portion).

Automatic—A firearm design that feeds cartridges, fires, extracts

and ejects cartridge cases as long as the trigger is fully depressed

and there are cartridges in the feed system. Also called “full

auto” and “machine gun.”

Revolver—A firearm, usually a handgun, with a cylinder having

several chambers so arranged as to rotate around an axis and be

discharged successively by the same firing mechanism. See also

“revolver.”

Lever—A design wherein the breech mechanism is cycled by an

external lever generally below the receiver.

Slide—An action that features a movable forearm which is

manually actuated in motion parallel to the barrel by the shooter.

Forearm motion is transmitted to a breech bolt assembly that

performs all the functions of the firing cycle assigned to it by the

design. 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 abutments

usually in the receiver. There are two principal types of bolt

actions: the turn bolt and the straight pull.

Bolt Face

See Breech Face

Bore

The interior of a barrel forward of the chamber.

Breech Face

That part of the breechblock or breech bolt which is against the

head of the cartridge case or shotshell during firing.

APPENDIX & GLOSSARY
Bullet

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

Cartridge

A single unit of ammunition consisting of the case, primer, and

propellant with one or more projectiles. Also applies to a

shotshell.

Cartridge, Center Fire

Any cartridge that has its primer central to the axis in the head

of the case.

Cartridge, Rimfire

A flange-headed cartridge containing the priming mixture inside

the rim cavity.

Cartridge Case

The container for all the other components that comprise a

cartridge.

Chamber

The rear part of the barrel bore that has been formed to accept a

specific cartridge. Revolver cylinders are multi-chambered.

Chamber Marks

Individual microscopic marks placed upon a cartridge case by

the chamber wall as a result of any or all of the following: (1)

chambering (2) expansion during firing (3) extraction.

Class Characteristics

Measurable features of a specimen that indicates a restricted

group source. They result from design factors and are therefore

determined prior to manufacture.

Comparison Microscope

Essentially two microscopes connected to an optical bridge that

allows the viewer to observe two objects simultaneously with the

same degree of magnification. This instrument can have a

monocular or binocular eyepiece. Sometimes referred to as a

“comparison macroscope.”

Deep Hole Drilling

A modern technique for barrel drilling involving rotation of the

blank on a nonrotating bit, under high pressure lubrication. Also,

an operation in which the depth of the hole is 10 or more times

greater than the diameter of the drill.

Ejector

A portion of a firearm’s mechanism thatejects or expels

cartridges or cartridge cases from a firearm.

Extractor

A mechanism for withdrawing the cartridge or cartridge case

from the chamber.

Firearm

An assembly of a barrel and action from which a projectile is

propelled by products of combustion.

Firing Pin

That part of a firearm mechanism that strikes the primer of a

cartridge to initiate ignition. Sometimes called “hammer nose”

or “striker.”

Function Testing

The examination of a firearm concerning its mechanical

condition and operation. It is usually performed to determine if

all safety features are operable and/or if the firearm is capable of

firing a cartridge.

Groove

See “rifling.”

Handgun

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

Impression

Contour variations on the surface of an object caused by a

combination of force and motion where the motion is

approximately perpendicular to the plane being marked. These

marks can contain “class” and/or “individual characteristics.”

Individual Characteristics 

Marks produced by the random imperfections or irregularities of

tool surfaces. These random imperfections or irregularities are

produced incidental to manufacture and/or caused by use,

corrosion, or damage. They are unique to that tool and

distinguish it from all other tools.

Land

The raised portion between the grooves in a rifled bore.

Magazine

A container for cartridges that has a spring and follower to feed

those cartridges into the chamber of a firearm. The magazine

may be detachable or an integral part of the forearm.

Obturation

1. The sealing of gases due to the expansion of a cartridge case as

a result of chamber pressure. 

2. The sealing of gases due to the expansion and/or upset of the

bullet base as it travels down the bore.

Pistol

A handgun in which the chamber is part of the barrel. A term

sometimes used for “handgun.”

Primer

The ignition component of a cartridge.

3 2 FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY
Projectile

An object propelled by the force of rapidly burning gases or

other means.

Propellant

In a firearm, the chemical composition, which when ignited by a

primer, generates gas. The gas propels the projectile. Also called

“powder”; “gunpowder”; “smokeless powder.”

Range of Conclusions Possible When Comparing

toolmarks

The examiner is encouraged to report the objective observations

that support the findings of toolmark examinations. The

examiner should be conservative when reporting the significance

of these observations.

• Identification 

Agreement of a combination of individual characteristics and

all discernible class characteristics where the extent of

agreement exceeds that which can occur in the comparison of

toolmarks made by different tools and is consistent with the

agreement demonstrated by toolmarks known to have been

produced by the same tool.

• Inconclusive

A. Some agreement of individual characteristics and all

discernible class characteristics, but insufficient for an

identification.

B. Agreement of all discernible class characteristics without

agreement or disagreement of individual characteristics due

to an absence, insufficiency, or lack of reproducibility.

C. Agreement of all discernable class characteristics and

disagreement of individual characteristics, but insufficient

for an elimination.

• Elimination

Significant disagreement of discernable class characteristics

and/or individual characteristics.

• Unsuitable

Unsuitable for examination.

Reamer

One of many spiral or straight-fluted multi-edged cutting tools

used to size and shape a hole.

Revolver

A firearm, usually a handgun, with a cylinder having several

chambers so arranged as to rotate around an axis and be

discharged successively by the same firing mechanism. 

Rifle

A firearm having rifling in the bore and designed to be fired

from the shoulder.

Rifling

Helical grooves in the bore of a firearm barrel to impart rotary

motion to a projectile.

Rifling Methods

Broach, Gang—A tool having a series of cutting edges of slightly

increasing height used to cut the spiral grooves in a barrel. All

grooves are cut with a single pass of the broach. 

Broach, Single—A non-adjustable rifling cutter which cuts all of

the grooves simultaneously, and is used in a series of increasing

dimensions until the desired groove depth is achieved. 

Button—A hardened metal plug with a rifled cross section

configuration. It is pushed or pulled through a drilled and

reamed barrel so as to cold form the spiral grooves to the desired

depth and twist. When the carbide button was first introduced it

was described as a “swaging process” or “swaged rifling.”

Hook—A cutting tool that has a hook shape and only cuts one

groove at a time. 

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

Swage—An internal mandrel with rifling configuration that

forms rifling in the barrel by means of external hammering. Also

known as “hammer forging.”

Rimfire

See “cartridge, rimfire.”

Shotgun

A smooth bore shoulder firearm designed to fire shotshells

containing numerous pellets or sometimes a single projectile.

Shoulder

1. 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.

Striations

Contour variations, generally microscopic, on the surface of an

object caused by a combination of force and motion where the

motion is approximately parallel to the plane being marked.

These marks can contain “class” and/or “individual

characteristics.”

Striker

A rod-like firing pin or a separate component that impinges on

the firing pin.

FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY 3 3
Subclass Characteristics

Discernible surface features of an object that are more restrictive

than “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 it

Relates to toolmarks

• The theory of identification as it pertains to the comparison of

toolmarks enables opinions of common origin to be made

when the unique surface contours of two toolmarks are in

“sufficient agreement.”

• This “sufficient agreement” is related to the significant

duplication of random toolmarks as evidenced by the

correspondence of a pattern or combination of patterns of

surface contours. Significance is determined by the

comparative examination of two or more sets of surface

contour patterns comprised of individual peaks, ridges and

furrows. Specifically, the relative height or depth, width,

curvature and spatial relationship of the individual peaks, ridges

and furrows within one set of surface contours are defined and

compared to the corresponding features in the second set of

surface contours. Agreement is significant when it exceeds the

best agreement demonstrated between toolmarks known to

have been produced by different tools and is consistent with

agreement demonstrated by toolmarks known to have been

produced by the same tool. The statement that “sufficient

agreement” exists between two toolmarks means that the

agreement is of a quantity and quality that the likelihood

another tool could have made the mark is so remote as to be

considered a practical impossibility.

• Currently the interpretation of individualization/identification

is subjective in nature, founded on scientific principles and

based on the examiner’s training and experience. 

Tool

An object used to gain mechanical advantage. Also thought of as

the harder of two objects that, when brought into contact with

each other, results in the softer one being marked.

3 4 FIR E ARM ID E N T I F IC AT I O N I N T H E FOR E N S IC SC I E NC E LAB OR AT ORY

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