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The following statement regarding the science involved in the discipline of forensic toolmark

examinations and conclusions is the opinion of the Scientific Working Group for Firearms and

Toolmarks (SWGGUN). The SWGGUN is a professional association of international scientific

experts in the forensic science discipline of Toolmark Identification with the mission of providing

a consensus of guidelines, studies, and other findings of interest to the forensic community.

The Foundations of Firearm and Toolmark Identification

Abstract: In 2005, the SWGGUN formed a Science Committee that was tasked

to conduct a comprehensive review of all pertinent and related documents in

the toolmark identification discipline in order to determine the reliability of

their conclusions in judicial proceedings. During the course of this review, the

SWGGUN Science Committee also examined the scientific rudiments upon

which this discipline is based. The purpose of this study was to chronicle the

origins and evolution of the discipline of firearm and toolmark identification as

a science.

SCIENTIFIC RUDIMENTS

Regardless of the branch of knowledge being investigated, science always involves systematic

procedures to gather reliable and testable knowledge. Depending on the specific realm of

scientific inquiry, a variety of procedures may be used. Therefore, it is important to establish

the context for the discussion by developing a common understanding of the procedures

underlying the discovery, origination and development of firearm and toolmark identification as

a science.

In this report, the Committee relied on Richard Feynman’s writings [1] on the character of

physical laws for guidance in articulating the critical scientific method elements used in the

discovery, origination, and evolution of firearm and toolmark identification. Feynman describes

these principles and empirical processes of discovery and/or demonstrated characteristics

necessary for scientific investigation as 1) observation of phenomena, 2) formation of a

hypothesis concerning the observed phenomena 3) experimentation to demonstrate truth or

falseness of the hypothesis, and 4) a conclusion that validates or modifies the hypothesis into a

supported theory.

The following scientific method elements consistent to those rudiments described by Feynman

have been used in establishing the forensic discipline of firearm and toolmark identification as a
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science: 1) observation of a phenomenon, 2) developing a premise, forming a hypothesis, 3)

develop a testing model, 4) using reliable methodology, and 5) forming a theory. The

application of these elements in the origination and development of the forensic discipline of

firearm and toolmark identification are delineated below.

PHENOMENON - The first documentation noting the observation of toolmarks was published in

the Buffalo Medical Journal in 1905. Concurrent with this were observations being made by

scientists in the United States, Germany, and France.

PREMISE - Tools have and impart distinctive features that can distinguish, or be traced back to,

that particular tool. This is referred to as the Premise of Uniqueness by Dr. Paul Kirk [2].

HYPOTHESIS - Tools have distinctive features, called individual characteristics, which are

random in nature. These individual characteristics enable a properly trained and competent

examiner to trace a toolmark back to the tool that produced it with great accuracy.

TESTING MODEL - The testing model used for over 50 years is the study of toolmarks produced

from consecutively manufactured tools where there would be the highest possibility of

toolmark carryover (e.g., two tools possessing and imparting indistinguishable toolmarks).

METHODOLOGY - Side-by-side comparison that allows analysis of questioned toolmark features

to known source toolmark features in a direct (one-to-one) spatial relationship to each other.

This is commonly referred to pattern recognition and was adapted from the textile industry.

THEORY - A conclusion of common origin, between two compared toolmarks, can be made

when there is sufficient correspondence of distinctive and individual features. A properly

trained and competent examiner can trace a toolmark back to the tool that produced it with

great accuracy.

Foundational Overview of Firearm and Toolmark Identification as a Science

Toolmark Identification is a forensic science discipline that is typically referred to as Firearm

and Toolmark Identification because a firearm is considered a specialized tool and the most

common tool examined by forensic science laboratories. Congruous with the elements of

scientific inquiry as described under Scientific Rudiments, toolmark identification is derived

from previously established scientific principles from recognized disciplines of physical science

to explain certain observed phenomena. And, as such, is considered an applied science rather

than a pure or basic science.
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The basis for identification in firearm and toolmark identification is founded on the principle of

uniqueness as described by Kirk [2] through Tuthill [3]; wherein, all objects are unique to

themselves and thus can be differentiated from one another. With respect to tools and

toolmarks, when the surface of a harder object (the tool) comes into contact with a softer

object (the work piece), the harder object will impart its unique marks or features on the softer

object thereby enabling a trained examiner to identify the source of a toolmark by comparing

known toolmarks produced by that tool. In firearm and toolmark identification it is important to

understand not only the practical elements of performing a comparison but also how tools are

manufactured and the mechanics by which toolmarks are made.

The mechanism for the origination of toolmarks is founded on well-established principles

derived from the physical sciences that include physics, metallurgy, metallography, and

materials science, as well as many mechanical properties presently used in mechanical and

industrial engineering. The working edge(s) of tools (those surfaces that come into contact with

the work piece and responsible for leaving toolmarks) are manufactured by a number of

machining methods. Most manufacturing processes involve the transfer of rapidly changing or

random marks onto work pieces such as barrel bores, breechfaces, firing pins, screwdriver

blades, and the working surfaces of other common tools. This is caused principally by the

phenomena of tool wear and chip formation or by electrical/chemical erosion. Microscopic

marks on tools may then continue to change from further wear or abuse. These irregularities

are considered unique and capable of being used to individualize or distinguish one tool from

another.

When the working edges of these tools come into contact with a work piece, these random

irregularities may be transferred to the work piece in the form of a toolmark. A comparative

examination of toolmarks may then permit a trained examiner to determine whether the

toolmarks share a common source. The comparative examination process used in firearm and

toolmark identification is similar to those used in the other comparative disciplines in forensic

science. This process begins with a study of the most general characteristics (class) of items to

be compared, progressing through (subclass, if present) to the analysis and comparison to the

most specific characteristics (individual). The most widely accepted method used in conducting

such an examination is a side-by-side microscopic comparison of the markings on a questioned

material item to known source marks imparted by a tool.

A number of studies have been published that examine various manufacturing methods and the

uniqueness of marks produced on the tool working surfaces. Of these, some of the most

valuable are those that have examined consecutively manufactured tools. The reason is that if

any tool working surfaces should appear most similar and difficult to distinguish from one
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another, it would be those that were manufactured consecutively [4, 5, 6, 7, 8, 9, 10, 11, 12, 13,

14, 15, 16]. These studies have demonstrated that sufficient uniqueness is produced on tool

working surfaces such that toolmarks produced by these tool working surfaces could be

correctly associated to the tool that produced the toolmark.

Years of empirical and experimental testing has allowed for the formation of a standard by

which trained examiners can form an opinion of common source when performing a

comparative examination. This testing includes the aforementioned studies of consecutively

manufactured tools, as well as, comparing the theoretical and demonstrated probabilities of

finding patterns of striae within a toolmark. Several experiments comparing the number of

matching striae or patterns of striae within toolmarks using a statistical approach have been

documented [17,18,19,20,21].

The most recent, although certainly not new, area of research involves machine-based analyses

of toolmarked surfaces. To date, the focus of this research has involved computerized systems

that analyze the similarity of surfaces marked by the same tool and those marked by different

tools [22, 23, 24, 25, 26, 27, 28]. The results of these experiments have shown conclusively that

a machine can detect significant differences between known matches and known non-matches

[29].

These empirical and experimental studies, based on the fundamental proposition that different

tool working surfaces will display sufficient uniqueness to permit the identification of a

toolmark produced by a tool to the correct source, have allowed for the formation and

continued testing of the Association of Firearm and Tool Mark Examiners (AFTE) Theory of

Identification. “The Theory of Identification as it pertains to the comparison of toolmarks

enables opinions of common origin to be made when unique surface contours of two toolmarks

are in ‘sufficient agreement’. Agreement is significant when the agreement in individual

characteristics exceeds the best agreement demonstrated between toolmarks known to have

been produced by different tools and is consistent with the agreement demonstrated by

toolmarks known to have been produced by the same tool.”[30] Derived from principles in

established physical science disciplines, the theories and hypothetical propositions described

were formed and tested to validate or falsify the science of firearm and toolmark identification.

Through on-going model testing, the AFTE Theory of Identification has been continually

validated and to date has never been disproven.

Any individual association or identification conclusion effected through this examination

process is based not on absolute certainty, but rather on the practical certainty of the

underlying (validated) scientific theory. The significance of identifications in this context can be
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evaluated in one of two ways. These include statistics to aid in the development of random

match probabilities and validation testing of trained examiners.

Studies that have assessed statistical approaches and probabilities have demonstrated that

toolmark identifications are reliable. However, using such an approach is not practical in

casework and has significant limitations. Probabilities do not describe cause and effect

relationships, but rather demonstrate the chances of a particular event happening in general

based on observations of a limited population. While statistics can lend an order to the

estimation of uncertainty, its utility for a particular case is limited in that aspect. In addition,

mathematical methods require some form of quantification of observed individual random

markings. Such quantification has been modeled and, for striated toolmarks, has achieved some

level of codification [17, 31, 32, 33, 34, 35]. While continuing study is taking place for both

mechanical and computerized models for quantification, such methods are not routinely used

in casework.

The other manner in which meaning can be placed on the significance of an identification is

through validation testing and assessment of error rates. In such testing, a trained examiner is

challenged with the comparative examination of toolmarks in an attempt to determine whether

the toolmarks share a common source. Due to the nature of the examination, a subjective

interpretation of observations made during the examination process of these validation studies

can provide for a reliable estimate of how often, using accepted procedures and methods, a

trained examiner will make an error in the determination of common source [36, 37, 38, 39, 40,

41, 42, 43, 44, 45, 46]. All of these studies help to demonstrate the validity of firearm and

toolmark identification and show that despite the subjective nature of the analysis, competent

examiners will rarely, if ever, commit false identifications or false eliminations.

Given the endless variety of tools and firearms and the occasional development of new

manufacturing methods, this effort, in principle, could be expanded considerably. Having an

entity outside the forensic laboratory prepare and administer each test could maximize

objectivity, allow for testing in multiple laboratories, and minimize the workload on laboratory

examiners. In addition to providing useful information about errors, validity-test results can be

used to evaluate current identification and elimination standards, while helping to identify

possible obstacles in the field such as the discovery of significant subclass marks produced by a

specific make and model tool.

Toolmark Identification is a Forensic Science

The science of firearm and toolmark identification is an applied science most often utilized for

forensic purposes to aid the criminal justice system for presentation to a court of law. Over the
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course of time, many sciences have been developed for answering questions disputed in

adversarial judicial systems around the world. To guard against the possibility of fraudulent

testimony, several courts have developed criteria for the acceptance or rejection of the

presentation of expert testimony. That standard of acceptance has varied and evolved over the

years and relies greatly on judges (with or without a scientific background) to make the final

decision as to the relevance and reliability of the testimony in question. To date, firearm and

toolmark identification has stood the test of judicial scrutiny and has served communities

around the world in solving crime.

CONCLUSION

During this study the Science Committee found that Firearm and Toolmark Identification:

 Originated and evolved into an applied scientific discipline following the general

precepts of scientific inquiry.

 Was founded on a sound scientific method that applied a logical premise to

explain noted phenomena.

 From this premise, a hypothesis, as well as, testing model and methodology

were developed.

 The underlying theory and methodology is continually tested (by scientists in and

out of discipline) and validated.

Based on the information provided above that summarizes sound foundational principles,

logical basis of identification, methodologies, established protocols and practices longaccepted, it is the conclusion of the Scientific Working Group for Firearms and Toolmarks

(SWGGUN) that the discipline of Firearms/Toolmark Identification is scientific and reliable.

Concomitantly, the identifications, individual associations or “matches” effected in this

discipline have firm scientific grounding with an extremely high degree of reliability based on

the practical certainty of the validated theory.

The SWGGUN concludes that sufficient validation testing by competent examiners and

collaborating scientists have been conducted to affirm the theory of firearm and toolmark

identification over the past ninety years for it to be considered a legitimate science pursuant to

the criteria set forth in the scientific method. At the same time, the SWGGUN also is cognizant

of the need for continual testing of the underlying theory, the scrutiny of the employed

methods and procedures, along with the continual awareness of emerging technologies that

could be used to further these endeavors.
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1

Feynman RP. The Character of Physical Laws. MIT Press, 1967. Feynman's writings on the

character of physical laws that include discussions of scientific inquiry, the order of nature,

physical science, theory to law and model development.

2

Kirk, P. The Ontogeny of Criminalistics. The Journal of Criminal Law, Criminology and Police

Science, 54(2): 236.

3

Tuthill, H. Individualization: Principles and Procedures in Criminalistics. Lightning Powder Co.,

Salem, OR, 1994.

4

Churchman, J.A. The Reproduction of Characteristics in Signatures of Cooey Rifles. RCMP

Police Gazette 11(5): 46-56.

5 Murdock, J. A General Discussion of Gun Barrel Individuality and an Empirical Assessment of

the Individuality of Consecutively Button Rifled .22 Caliber Rifle Barrels. AFTE Journal 13(3): 84-

95.

6 Matty, W. A Comparison of Three Individual Barrels Produced from One Button Rifled Barrel

Blank. AFTE 17(3): 64-69.

7

Brundage, D. The Identification of Consecutively Rifled Gun Barrels. AFTE Journal 30(3): 438-

444.

8

Tulleners, F. and Hamiel, J. Sub Class Characteristics of Sequentially Rifled 38 Special S&W

Revolver Barrels. AFTE Journal 31(2): 117-122.

9

Lopez, L. and Grew, S. Consecutively Machined Ruger Bolt Faces. AFTE 32(1): 19-24.

10 Coffman, B. Computer Numerical Control (CNC) Production Tooling and Repeatable

Characteristics on Ten RemingtonModel 870 Production Run Breech Bolts. AFTE 35(1): 49-54.

11 Nichols, R. Firearm and Tool Mark Identification: The Scientific Reliability and Validity of the

AFTE Theory of Identification Discussed Within the Framework of a Study of Ten Consecutively

Manufactured Extractors. AFTE Journal 36(1): 67-88.

12 Lightstone, L. The Potential for and Persistence of Subclass Characteristics on the Breech

Faces of SW40VE Smith &Wesson Sigma Pistols. AFTE 42(4): 308-322.

13 Butcher, S. and Pugh, P. A Study of Marks Made by Bolt Cutters. Journal of the Forensic

Science Society 15: 115-126.

14 Cassidy, F.H. Examination of Toolmarks from Sequentially Manufactured Tongue-and-Groove

Pliers. Journal of Forensic Science 25(4): 796-809.

15 Eckerman, S. A Study of Consecutively Manufactured Chisels. AFTE Journal 34(4): 379-390.

16 Clow, C. Cartilage Stabbing with Consecutively Manufactured Knives: A Response to Ramirez

v. State of Florida. AFTE Journal 37(2): 86-116.

17 Biasotti, A. A Statistical Study of the Individual Characteristics of Fired Bullets. Journal of

Forensic Science, 4(1): 34-50.

18 Bracket, J.W. A Study of Idealized Striated Marks and Their Comparisons Using Models.

Journal of the Forensic Science Society, 10(1): 27-56.

19 Deinet, W. Studies of Models of Striated Marks Generated by Random Processes. Journal of

Forensic Sciences, 26(1): 35-50.

20 Stone, R. How Unique are Impressed Toolmarks? AFTE Journal 35(4): 376-383.

21 Collins, E. How “Unique” Are Impressed Toolmarks? – An Empirical Study of 20 Worn

Hammer Faces. AFTE Journal 37(4): 252-295.
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22 Bachrach. B. et al. A Statistical Validation of the Individuality and Repeatability of Striated

Tool Marks: Screwdrivers and Tongue and Groove Pliers. Journal of Forensic Sciences, 55(2):

348-357. In the abstract, the authors offer that, “An analysis of the resulting matching and

nonmatching similarity distributions [of striated toolmark images] shows nearly error-free

identification under most conditions. These results provide substantial support for the validity

of the premise of tool mark identification.”

23 Chumbley, L.S. et al. Validation of Tool Mark Comparisons Obtained Using a Quantitative,

Comparative, Statistical Algorithm. Journal of Forensic Sciences 55(4): 953-961.

24 Weller, T. et al. Confocal Microscopy Analysis of Breech Face Marks on Cartridge Cases From

10 Consecutively Manufactured Pistol Slides. Journal of Forensic Sciences, 57(4): 912-917.

25 Petraco, N., et al. Addressing the National Academy of Sciences’ Challenge: A Method for

Statistical Pattern Comparison of Striated Tool Marks. Journal of Forensic Sciences, 57(4): 900-

911.

26 Petraco, N., et al. Forensic Surface Metrology: Tool Mark Evidence. Scanning, 33:272-278.

27 Song, J. et al. Development of Ballistics Identification – From Image Comparison to

Topography Measurement in Surface Metrology. Measurement Science and Technology, 23.

28 Petraco, N. Application of Machine Learning to Toolmarks Statistically Based Methods for

Impression Pattern Comparisons. 2012 USDOJ 2009-DN-BX-K041.

29 With continued research and development, machines such as these could eventually be used

in laboratories for independent confirmations of examiners’ conclusions in casework and,

ultimately, may replace human judgment in the comparison process. Because the working

surfaces of different tools change at unpredictable rates, “ID machines” (or the identification

cut-off values inherent to these systems) would need to be fine-tuned to reach acceptable

trade-offs among false positives and negatives and sensitivity and specificity. This desired

balance will undoubtedly be the subject of some discussion and may require specific research

to justify whatever balance is ultimately selected. Clearly, machines will not find wide

acceptance unless and until the overall quality of their output can be demonstrated to match or

exceed those of human examiners.

30 AFTE, Committee for the Advancement of the Science of Firearm and Toolmark Identification.

Theory of Identification as it Related to Toolmarks: Revised. AFTE Journal 43(4): 287.

31 Biasotti, A. and Murdock, J. Criteria for Identification or State of the Art of Firearm and

Toolmark Identification. AFTE Journal, 16(4): 16-24.

32 Miller, J. and McLean, M. Criteria for Identification of Toolmarks. AFTE Journal 30(1): 15-61.

33 Miller, J. Criteria for Identification of Toolmarks Part II – Single Land Impression Comparisons.

AFTE Journal 32(2): 116-131.

34 Consecutive Matching Striations (CMS): Its Definition, Study and Application in the Discipline

of Firearms and Tool Mark Identification. AFTE Journal 35(3): 298-307.

35 Miller, J. and Neel, M. Criteria for Identification of Toolmarks Part III – Supporting the

Conclusion. AFTE Journal 36(1): 7-38.

36 DeFrance, C. and Van Arsdale, M. Validation Study of Electrochemical Rifling. AFTE Journal,

35(1): 35-37.
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37 Bunch, S. and Murphy, D. A Comprehensive Validity Study for the Forensic Examination of

Cartridge Cases. AFTE Journal, 35(2): 200-203.

38 Thompson, E. and Wyant, R. Knife Identification Project (KIP). AFTE Journal, 35(4): 366-370.

39 Smith, E. Cartridge Case and Bullet Comparison Validation Study With Firearms Submitted in

Casework. AFTE Journal, 36(4): 130-135.

40 Giroux, B. Empirical and Validation Study: Consecutively Manufactured Screwdrivers. AFTE

Journal, 41(2): 153-158.

41 Hamby, J., Brundage, D., Thorpe, J. The Identification of Bullets Fired from 10 Consecutively

Rifled 9mm Ruger Pistol Barrels: A Research Project Involving 507 Participants from 20

Countries. AFTE Journal, 41(2): 99-110.

42 Lyons, D. The Identification of Consecutively Manufactured Extractors. AFTE Journal, 41(3):

246-256.

43 Nichols, R. Validation Study for Impressed Toolmarks. Presented at the AFTE 41st Annual

Training Seminar, Las Vegas, NV May 4, 2010.

44 Fadul. T. et al. An Empirical Study to Evaluate the Repeatability and Uniqueness of

Striations/Impressions in Fired Cartridge Cases in 10 Consecutively Manufactured Slides. Final

Report, Award Number 2009-DN-BX-K230, 2011.

45 Mayland, B. and Tucker, C. Validation of Obturation Marks in Consecutively Reamed

Chambers. AFTE Journal, 44(2): 167-169.

46 Stroman, A. Empirically Determined Frequency of Error in Cartridge Case Examinations Using

a Double-Blind Format. Presented at the AFTE 43st Annual Training Seminar, Buffalo, NY, June

26, 2012.