2

Standard Scale of Source Conclusions and Criteria for Toolmark Examinations

Standard Scale of Source 

Conclusions and Criteria 

for Toolmark Examinations

Firearms and Toolmarks Subcommittee

Physics/Pattern Interpretation Scientific Area Committee

Organization of Scientific Area Committees (OSAC) for Forensic Science
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Standard Scale of Source Conclusions and Criteria for Toolmark Examinations

OSAC Proposed Standard

Standard Scale of Source Conclusions 

and Criteria for Toolmark 

Examinations

Prepared by 

Firearms & Toolmarks Subcommittee

Version: 1.0

Disclaimer:

This document has been developed by the Firearms & Toolmarks Subcommittee of the Organization 

of Scientific Area Committees (OSAC) for Forensic Science through a consensus process and is 

proposed for further development through a Standard Developing Organization (SDO). This 

document is being made available so that the forensic science community and interested parties can 

consider the recommendations of the OSAC pertaining to applicable forensic science practices. The 

document was developed with input from experts in a broad array of forensic science disciplines as 

well as scientific research, measurement science, statistics, law, and policy. 

This document has not been published by a SDO. Its contents are subject to change during the 

standards development process. All interested groups or individuals are strongly encouraged to 

submit comments on this proposed document during the open comment period administered by the 

Academy Standards Board (http://www.asbstandardsboard.org/).

 
4

Standard Scale of Source Conclusions and Criteria for Toolmark Examinations

Foreword

This standards document was proposed by the Firearms and Toolmarks Subcommittee of 

the Organization of Scientific Area Committees (OSAC) by submitting a request to the 

American Academy of Forensic Sciences (AAFS) Academy Standards Board (ASB). This 

document is intended to provide a standardized scale of source conclusions and underlying 

criteria for use by forensic firearm and toolmark examiners.

Additional documents which contain information related to this standards document 

include:

● Standard for Supporting Documentation of Source Conclusions inToolmark 

Examinations

● Best Practice Recommendations for Peer Review of Source Conclusions inToolmark 

Examinations

● Best Practice Recommendations for Expression of Source Conclusions in Toolmark 

Examinations

Keywords: firearm and toolmark source conclusions, scale of conclusions, criteria

This document was developed to provide a standard scale of conclusions and criteria to be 

used for all microscopic toolmark examinations and comparisons by forensic firearm and 

toolmark examiners.

All hyperlinks and web addresses shown in the document are current as of the publication date of 

this draft standard.
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Standard Scale of Source Conclusions and Criteria for Toolmark Examinations

Acknowledgements

Editor:

Deputy Editor(s):

Drafting Working Group Members:

Eric Collins Contra Costa County Office of theSheriff 

Nancy McCombs California Department of Justice

Max Morris Iowa State University

John Murdock John Murdock & Associates – ForensicConsultants 

Douglas Murphy Federal Bureau of Investigation

Michael Neel Bureau of Alcohol, Tobacco, Firearms andExplosives 

Ronald Nichols Nichols Forensic Consulting

Gerhard Wevers Institute of Environmental Scienceand Research, New Zealand 

Consensus Group Members:
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Standard Scale of Source Conclusions and Criteria for Toolmark Examinations

Table of Contents

1 Scope 5

2 Normative References 5

3 Terms and Definitions 5

4 Requirements 6

4.1 Value Determinations 6

4.1.1 No Value (Unsuitable) for Source Conclusion 6

4.1.2 Of Value for Source Conclusion 6

4.2 Scale of Source Conclusions and Related Criteria 6

4.2.1 Exclusion 6

4.2.1.1 Criteria for Exclusion 6

4.2.2 Insufficient Support for Exclusion 8

4.2.2.1 Criteria for Insufficient Support for Exclusion 8

4.2.3 Insufficient Support for Either Exclusion or Identification 8

4.2.3.1 Criteria for Insufficient Support for Either Exclusion or Identification 8

4.2.4 Insufficient Support for Identification 8

4.2.4.1 Criteria for Insufficient Support for Identification 8

4.2.5 Identification 9

4.2.5.1 Criteria for Identification 9

5 Qualifications and Limitations 10

Annex A 12

Annex B (Bibliography) 11
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Standard Scale of Source Conclusions and Criteria for Toolmark Examinations

1 Scope

This document provides a standard scale of conclusions and criteria to be used for all microscopic 

firearm and toolmark examinations and comparisons conducted for the forensic purpose of 

determining if two or more toolmarks were or could have been created by the same tool. This 

document takes into consideration the current state of professional practices and scientific 

research1. The scope of this document is limited to the process of developing source conclusions and 

does not address or consider other types of conclusions possible in the analysis of firearm and 

toolmark evidence. Throughout this document, the term “toolmark” is used to refer to both firearmproduced and non-firearm toolmarks.

2 NormativeReferences

3 Terms and Definitions

For the purposes of this document, the following definitions and abbreviations apply:

3.1

Class Characteristics

Observable features of a specimen which indicate a restricted group source. They result from design 

decisions made by a manufacturer that are within acceptable manufacturing tolerances and are, 

therefore, determined prior to manufacture.

3.2

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.

3.3

Known Matching Toolmarks 

KM

Toolmarks known to have been made by the same tool.

3.4

Known Non-Matching Toolmarks 

KNM

Toolmarks known to have been made by different tools or different working surfaces of the same

1 Numerous scientific studies have demonstrated the ability of firearm and toolmark examiners to make 

reliable and accurate source conclusion judgements; some of these studies are listed as references 3 through 9, 

11, 13 through 16, 23, 24, and 28. A listing and summary of additional supportive research and validation 

studies pertaining to non-firearm toolmarks can be found in the SWGGUN Admissibility Resource Kit (ARK).

https://afte.org/resources/swggun-ark/testability-of-the-scientific-principle.
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Standard Scale of Source Conclusions and Criteria for Toolmark Examinations

tool.

3.5

Subclass Characteristics

Toolmarks produced by a single tool during the manufacturing process that repeat virtually 

unchanged from the same tooling operation on a series of sequentially-manufactured items. These 

types of characteristics are not determined prior to manufacture and may originate from a source 

that is more restrictive (i.e., a subset) than that of the overall class to which they belong.

4 Requirements

4.1 Value Determinations

4.1.1 No Value (Unsuitable) for Source Conclusion

A judgement that the item under consideration is inadequate to form any source conclusion due to 

insufficient quality or quantity of features, size, damage, or clarity of the item (i.e., any object that 

does not bear any class, subclass and/or individual toolmarks of value for source conclusion).

However, the item may have value to other paths of forensic inquiry (e.g., crime scene 

reconstruction).

4.1.2 Of Value for Source Conclusion

A preliminary judgement that the item under consideration has potentially sufficient class, subclass 

and/or individual characteristics for further evaluation, examination, or comparison with other 

known-source or questioned-source items for potential source conclusion.

4.2 Scale of Source Conclusions and Related Criteria

4.2.1 Exclusion

An expert opinion that two items of toolmark evidence were marked by different tools. Source 

exclusion is the strongest statement of non-association expressed in forensic firearm and toolmark 

examination.

4.2.1.1 Criteria for Exclusion

An Exclusion is justified when the observed characteristics of the items in question provide 

extremely strong support for the proposition that they were marked by different tools and 

extremely weak or no support for the proposition that the two were marked by the same tool. This 

conclusion is based on 1) demonstrable differences in class, subclass or individual characteristics, 2) 

task-relevant information, and 3) the cumulative results of training and casework examinations that 

have either been performed, peer reviewed, and/or published in peer-reviewed journals.

4.2.1.1.1 An Exclusion shall be reached when there is a clear, demonstrable incompatibilityin
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Standard Scale of Source Conclusions and Criteria for Toolmark Examinations

class characteristics between the items in question. A source exclusion may only be 

expressed as a certainty if it is physically impossible (i.e., zero probability) for the 

examined items to have been marked by the same source tool based on an 

incompatibility in class characteristics.

4.2.1.1.2 If the class characteristics are compatible or possibly compatible, source exclusion is 

only justified if it is the examiner’s opinion that there are demonstrable differences in 

individual or subclass characteristics such that the excluded toolmarks fall outside 

the range of variability of marks produced by the same tool (KM) and are consistent 

with the amount of disagreement demonstrated by toolmarks known to have been 

produced by different tools (KNM).

The following task-relevant factors should be considered when determining if 

differences observed in the comparison of two toolmarks are sufficient for exclusion 

[18]:

If a suspect tool is available for examination:

● Evidence of alteration to the tool working surface

● Ability of the tool to consistently reproduce individual characteristics

● Condition of the tool working surface or substrate

● Relative hardness of the tool working surface or substrate

● Any known time interval between deposition of questioned toolmark(s) and 

collection of the tool, during which changes to the tool could have occurred 

due to use, abuse, or corrosion2

● The history of the tool, to the extent it can be established2

If a suspect tool is unknown or otherwise unavailable for examination:

● Time interval between the production or collection of the questioned 

toolmarks, if related to different events

● The quantity and quality of any additional questioned toolmarks available for 

analysis, to the extent it can be determined that they represent a reliable 

range of variability of individual characteristics arising from the same source 

tool

o Example: A group of four questioned bullets that can be identified as 

having been fired from the same unknown firearm based on consistently 

reproduced individual characteristics in the rifling impressions is 

compared to a bullet having the same class characteristics but displaying

2 For the purpose of determining if an Exclusion is warranted based on differences in individual 

characteristics, investigative details relating to the possible use or non-use of the suspected tool during the 

time interval between the criminal incident and the collection of the tool as evidence may be contextual taskrelevant information because it may help the examiner draw an accurate forensic conclusion [18].
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Standard Scale of Source Conclusions and Criteria for Toolmark Examinations

almost no agreement of individual characteristics with the 

aforementioned group; in this example, the fifth bullet could justifiably be 

excluded (per the criteria in 4.2.1.1.2) as having been fired from the same 

firearm that marked the group of four bullets, if it can be assumed there 

are no mitigating factors (e.g., a lengthy time interval between the crime 

scenes, difference in ammunition) that could possibly account for the 

observed disagreement.

4.2.2 Insufficient Support for Exclusion

An expert opinion that the observed similarities and differences in characteristics of the items in 

question are insufficient for conclusive exclusion but provide substantial support for the proposition 

that the two items were marked by different tools and weak support for the proposition that they 

were marked by the same tool.

4.2.2.1 Criteria for Insufficient Support for Exclusion

An Insufficient Support for Exclusion conclusion is justified when, in the examiner’s opinion, there is 

agreement of all discernible class characteristics and some disagreement of individual 

characteristics similar to that which has been demonstrated by KNM toolmarks, but insufficient for 

Exclusion (as previously defined in 4.2.1).

4.2.3 Insufficient Support for Either Exclusion or Identification

An expert opinion that the observed similarities and differences in characteristics of the items in 

question neither substantially support nor substantially refute the proposition that the items were 

marked by the same source tool.

4.2.3.1 Criteria for Insufficient Support for Either Exclusion or Identification

An Insufficient Support for Either Exclusion or Identification conclusion is justified when, in the 

examiner’s opinion, there is agreement of all discernible class characteristics, but, due to an absence, 

insufficient agreement and/or disagreement, or lack of reproducibility of individual characteristics, 

no other conclusion can be reached.

4.2.4 Insufficient Support for Identification

An expert opinion that the observed similarities and differences in characteristics of the items are 

insufficient for conclusive identification, but provide substantial support for the proposition that the 

two items were marked by the same tool and weak support for the proposition that the two were 

marked by different tools.

4.2.4.1 Criteria for Insufficient Support for Identification

An Insufficient Support for Identification conclusion is justified when, in the examiner’s opinion,
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Standard Scale of Source Conclusions and Criteria for Toolmark Examinations

there is agreement of all discernible class characteristics and some agreement of individual 

characteristics but not exceeding the best KNM and is therefore insufficient for Identification.

4.2.5 Identification

An expert opinion that two items of toolmark evidence were marked by the same tool. Source 

identification is the strongest statement of association expressed in forensic firearm and toolmark 

examination.

4.2.5.1 Criteria for Identification

An Identification conclusion is justified when, in the examiner’s opinion, the observed similarities in 

characteristics of the items in question provide extremely strong support for that proposition and 

negligible support for the proposition that the two items were marked by different tools.

In this context, “negligible support” means that based on 1) known empirical research and validation 

studies, and 2) the cumulative results of training and casework examinations that have either been 

performed, peer reviewed, or published in peer-reviewed scientific literature, it is the examiner’s 

opinion that it is extremely unlikely any firearms or tools other than those identified are capable of 

producing marks exhibiting sufficient agreement for identification.

4.2.5.1.1 An Identification conclusion is based on an examiner’s determination that all 

discernible class and individual3 characteristics agree such that the extent of 

agreement exceeds that which has been demonstrated by toolmarks made by 

different tools (KNM) and is consistent with the agreement demonstrated by 

toolmarks known to have been made by the same tool(KM).

3 The use of the term “individual characteristics” here implies the marks being compared cannot be subclass 

characteristics. Subclass characteristics are manufactured toolmarks that sometimes repeat virtually 

unchanged from one manufactured item to another over a limited run of manufactured items. When these 

characteristics are present on or near the working surfaces of tools, it is possible for these toolmarks to be 

mistakenly interpreted as individual characteristics (thus resulting in the identification of a toolmark to a tool 

other than the one that produced the mark) if they are not carefully evaluated by the examiner. Therefore, 

subclass influences must be recognized so the toolmarks they produce can be appropriately considered. It is 

important to note that although subclass toolmarks may be present near the working surface of the tool, they 

may, either because of their position or manner in which the tool is normally used, have no influence on the 

individuality of toolmarks made by this working surface or edge.

Before rendering a conclusion regarding the individuality of an examined toolmark or tool, all examined 

toolmarks and tool working surfaces (if available) that are used as a basis for the conclusion must be evaluated 

for the possible presence of subclass characteristic influence. Reference [20] offers a practical description of 

how such evaluations can be conducted. Any agreement of microscopic characteristics that is observed 

between two toolmarks that can be attributed to the presence of potential subclass influences must not be 

used as the basis for a conclusion of identification.
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Standard Scale of Source Conclusions and Criteria for Toolmark Examinations

5 Qualifications and Limitations [27]

5.1 An examiner shall not assert that two toolmarks originated from the same source to the 

exclusion of all other sources. This may wrongly imply that an Identification conclusion is 

based upon a statistically derived or verified measurement or an actual comparison to all 

other toolmarks in the world, rather than an examiner’s expert opinion.

5.2 An examiner shall not assert that examinations conducted in the forensic firearms and 

toolmarks discipline are infallible or have a zero error rate.

5.3 An examiner shall not provide a conclusion that includes a statistic or numerical degree of 

probability except when based on relevant and appropriate data.
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Standard Scale of Source Conclusions and Criteria for Toolmark Examinations

Annex A

(informative)

Bibliography

1. AFTE Committee for the Advancement of the Science of Firearm & Toolmark Identification. 

“Theory of Identification as it Relates to Toolmarks: Revised.” AFTE Journal, Vol. 43, No. 4, 

Fall 2011, p. 287. 4

2. AFTE Criteria for Identification Committee. “Theory of Identification, Range of Striae 

Comparison Reports and Modified Glossary Definitions – an AFTE Criteria for Identification 

Committee Report.” AFTE Journal, Vol. 24, No. 2, April 1992, pp. 336-340.

3. Baldwin, D.P., Bajic, S.J., Morris, M., and D. Zamzow. “A Study of False-Positive And FalseNegative Error Rates in Cartridge Case Comparisons.” Ames Laboratory, USDOE, Technical 

Report #IS-5207 (2014).

4. Brundage, D.J. “The Identification of Consecutively Rifled Gun Barrels.” AFTE Journal, Vol.30,

No. 3 (1998): 438-44.

5. Bunch, S. G., and D. Murphy. "A Comprehensive Validity Study for the Forensic Examination 

of Cartridge Cases." AFTE Journal, Vol. 35, No. 2 (2003): 201-203.

6. Cazes, M. and J. Goudeau. “Validation Study Results from Hi-Point Consecutively

Manufactured Slides.” AFTE Journal, Vol. 45, No. 2 (2013): 175-177.

7. DeFrance, C.S., and M.D. Van Arsdale. “Validation Study of Electrochemical Rifling.” AFTE 

Journal, Vol. 35, No. 1 (2003): 35-7.

8. Fadul, T. G. "An Empirical Study to Evaluate the Repeatability and Uniqueness of 

Striations/Impressions Imparted on Consecutively Manufactured Glock EBIS Gun Barrels." 

AFTE Journal, Vol. 43, No 1 (2011): 37-44.

9. Fadul, T.G., Hernandez, G.A., Stoiloff, S., and S. Gulati. “An Empirical Study to Improve the 

Scientific Foundation of Forensic Firearm and Tool Mark Identification Utilizing 10 

Consecutively Manufactured Slides.” AFTE Journal. Vol. 45, No. 4 (2013): 376-93.

10. Faigman, D.L., Kaye, D.H., Saks, M.J., and Sanders, J. Modern Scientific Evidence: The Law and 

Science of Expert Testimony, Part II: Science and the Scientific Method, Ch. 4: Scientific 

Method: The Logic of Drawing Inferences From Empirical Evidence, Vol. 1. St Paul: West, 

2002.

4 Association of Firearm and Tool Mark Examiners, PO Box 414, Waterloo, IL 62298
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Standard Scale of Source Conclusions and Criteria for Toolmark Examinations

11. Hamby, J.E., Brundage, D.J., and J.W. Thorpe. “The Identification of Bullets Fired from 10 

Consecutively Rifled 9mm Ruger Pistol Barrels: A Research Project Involving507

Participants from 20 Countries.” AFTE Journal, Vol. 41, No. 2 (2009): 99-110.

12. Hatcher, J.S. Textbook of Firearms Investigation, Identification and Evidence. Marines, NC: 

Small Arms Technical Publishing Company, 1935, pp. 274-288.

13. Keisler, M. A., Hartman, S., Kilmon, A., Oberg, M., and Templeton, M. “Isolated Pairs Research 

Study.” AFTE Journal, Vol. 50, No. 1 (2018): 56-58.

14. Lyons, D. J. "The Identification of Consecutively Manufactured Extractors." AFTE Journal, Vol. 

41, No. 3 (2009): 246-256.

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

Chambers." AFTE Journal, Vol. 44, No. 2 (2012): 167-169.

16. Miller, J. “Criteria for the Identification of Tool Marks.” AFTE Journal, Vol. 30, No. 1,Winter

1998, pp. 15-61.

17. Moran, B. and Murdock, J. “Appendix No. 2: The Application of the Scientific Method to 

Firearm and Toolmark Examination.” In Grzybowski, R., Miller, J., Moran, B., Murdock, J.,

Nichols, R., and Thompson, R., “Firearm/Toolmark Identification: Passing the Reliability Test 

under Federal and State Evidentiary Standards.” AFTE Journal, 35(2), Spring 2003, pp. 234-

240.

18. National Commission on Forensic Science, Human Factors Subcommittee, Views Document: 

“Ensuring That Forensic Analysis Is Based upon Task-Relevant Information”, available from

https://www.justice.gov/archives/ncfs/file/795286/download.

19. Nichols, R. Firearm and Toolmark Identification: The Scientific Reliability of the Forensic 

Science Discipline. First Edition, Elsevier: Academic Press, 2018.

20. Nichols, R. “Subclass Characteristics: From Origin to Evaluation.” AFTE Journal, Vol. 50, No. 2,

Spring 2018, pp. 68-88.

21. Scientific Working Group for Firearms and Toolmarks (SWGGUN),“Elimination Factors 

Related To FA/TM Examinations.”

22. Scientific Working Group for Firearms and Toolmarks (SWGGUN), “The Foundations of

Firearm and Toolmark Identification,” 2013.

23. Smith, E. “Cartridge Case and Bullet Comparison Validation Study with Firearms Submitted 

in Casework.” AFTE Journal, Vol. 37, No. 2 (2005): 130-5.

24. Smith, T., Smith, G.A., Snipes, J.B. “A Validation Study of The Bullet and CartridgeCase
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Standard Scale of Source Conclusions and Criteria for Toolmark Examinations

Comparisons Using Samples Representative of Actual Casework.” Journal of Forensic

Sciences, Vol. 61, No. 4: 939-946.

25. Thompson, Robert M. “Firearm Identification in the Forensic Science Laboratory.”National

District Attorneys Association, 2010.

26. Thornton, John I. “Nonrandomness of Striation Evidence.” Journal of Police Science and

Administration, Vol. 6, No. 4, 1978, pp. 413-415.

27. U.S. Department of Justice. “Uniform Language for Testimony and Reports for the Forensic 

Firearms/Toolmarks Discipline – Pattern Match Examination.” Adopted 07/24/2018, 

Effective 01/24/2019, p. 3.

28. Stroman, A. “Empirically Determined Frequency of Error in Cartridge Case Examinations

Using a Declared Double-Blind Format.” AFTE Journal, Vol. 46, No. 2 (2014), pp. 157-175.

29. Vorburger, T., Song, J., Petraco, N., “Topography Measurements and Applications in Ballistics 

and Tool Mark Identifications.” Surface Topography: Metrology and Properties, 4(2016) 

013002, pp. 1-35.