The Scientific Working Group for Firearms and Toolmarks (SWGGUN) was formed in 1998 through funding by the National Institute of Justice. SWGGUN’s goals were to have subject matter experts provide working guidelines, discuss analytic methods, monitor research and technology, as well as exchange ideas and other information in the forensic firearm and toolmark discipline.

The Admissibility Resource Kit (ARK) was originally developed and published to the web in 2005 by members of the newly formed SWGGUN Daubert Committee to devise some type of training program/tool that could assist firearm examiners in better preparing for evidence admissibility hearings that began to greatly proliferate in 2002.

With the defunding of the SWG groups by the federal government in 2013 and its functions being incorporated into the NIST/DOJ Organization of Scientific Area Committees (OSAC), the AFTE Board of Directors and past members of SWGGUN decided to republish and maintain the ARK on the AFTE website.

The following information is a collection of information pertaining to the admissibility of firearm and toolmark evidence. The material presented here is the direct result of hard work, research and dedication of many scientists and it is posted here for you to read, evaluate and prepare yourself when educating the criminal justice system on this subject.

This collection of resources is not all-inclusive but rather represents significant research, legal opinions, challenges, rulings and other issues related to the discipline.


Admissibility standards being applied to forensic science disciplines are creating new challenges for the individual analysts when providing expert witness testimony.  The analysts are being held to a higher standard in justifying the science of their respective disciplines.

The Admissibility Resource Kit (ARK) is a repository of pertinent information designed to primarily assist Firearms and Toolmark Examiners in quickly preparing for evidence admissibility hearings. The information contained herein consists of general & foundational text, listed documents, related internet website links and visual aids that serve as an effective educational tool. The layered or nested formatting of this information will hopefully provide a progressive learning vehicle that will quickly educate the end-user on the critical elements that should be mastered to articulate the underlying scientific principles of the Firearms and Toolmark Identification forensic discipline.

The SWGGUN ARK is broken down into the following categories:

  • Admissibility Rules Overview
  • Foundational Overview of Firearm/Toolmark Identification
  • Review of Admissibility Elements
  • Court Rulings
  • Opposing and Supportive Viewpoints of Firearm and Toolmark Identification
  • Appendices

When available, the actual articles will be provided and can be accessed by clicking the title of the article.

A list of significant court decisions is provided here that address the admissibility of expert scientific evidence and testimony.


Frye Test

A 1923 decision by the District of Columbia Court of Appeals, known as the Frye test, set a foundation for the admissibility of expert scientific evidence.  Under the Frye test, such expert testimony was admissible only if the principles on which it was based had been "generally accepted" by the scientific community.

Rule 702 - Testimony by Experts    Notes

In 1973 (revised Dec. 1, 2003) Federal Rules of Evidence, Fed. R. Evid. 702, were adopted by the federal courts system.  This rule that broadly governed the admissibility of expert testimony, didn't mention general acceptance, but stated:

"If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise, if (1) the testimony is based upon sufficient facts or data, (2) the testimony is the product of reliable principles and methods, and (3) the witness has applied the principles and methods reliably to the facts of the case."


Daubert refers to Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993), a case in which the court outlined criteria which they believed could be important to establish the reliability of expert testimony. These criteria include the following:

  • Has the technique or theory been scientifically tested?
  • Does the technique or theory have a known or potential error rate?
  • Has the technique or theory been subjected to peer review and publication?
  • Is the technique or theory subject to standards governing its application?
  • Is the technique or theory generally accepted by the relevant scientific community?

Although these criteria were not meant as a checklist, they are being applied in this way. If one of these criteria is not met, then the testimony may be found to be inadmissible.


Expert evidence must be necessary in order to allow the fact finder: (1) to appropriate the facts due to their technical nature, or; (2) to for a correct judgment on a matter if ordinary persons are unlikely to do so without the assistance of persons with special knowledge.

R. v. Mohan [2 S.C.R. 9] 1994

Admission of expert evidence depends on the application of the following criteria:

  • relevance;
  • necessity in assisting the trier of fact;
  • the absence of any exclusionary rule;
  • a properly qualified expert

R. v. D.D. [Supreme Court of Canada] 2000

"The application of the four Mohan criteria is case-specific.  Determinations of relevance and necessity, as well as the assessment of whether the prejudicial effect of the evidence outweighs its probative value, must be made within the factual context of the trial."

Per Major J., 46 "The second requirement of the Mohan analysis exists to ensure that the dangers associated with expert evidence are not lightly tolerated.  Mere relevance or "helpfulness" is not enough.  The evidence must also be necessary."

"A fortiori, a finding that some aspects of the evidence "might reasonably have assisted the jury" is not enough.  As stated by Sopinka et al., expert evidence must be necessary in order to allow the fact finder: (1) to appreciate the facts due to their technical nature, or; (2) to form a correct judgment on a matter if ordinary persons are unlikely to do so without the assistance of persons with special knowledge."

R. v. J.L.J. [Supreme Court of Canada] 2000

The Supreme Court expressly referenced Daubert as a relevant authority [FN2-19 below] where the Supreme Court expressly referenced Daubert as a relevant authority and referred to many of the same factors for analysis referenced by the Daubert court.  The Canadian law has now been made clear by the Supreme Court.


No substantial standards for Europe have been found to date.

Toolmark Identification is a discipline of forensic science that is usually listed as Firearm and Toolmark Identification because a firearm is considered a specialized tool.

A toolmark examination is an empirical comparative study that can determine if a striated or impressed mark was produced by a particular tool.

The basis for identification in Toolmark Identification is founded on the principle of uniqueness as described by Kirk through Tuthill; wherein, all objects are unique to themselves and thus can be differentiated from one another. Additionally, the underlying mechanism for the origination of toolmarks is that when a harder object (the tool) comes in contact with a softer object (work piece), the harder object will impart its marks or features on the softer object.  This 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 edges of tools, that include components of firearms that contact ammunition, generally consist of some type of hard material, such as steel, to ensure strength and durability of the tool; while work pieces are generally made of softer materials. These surfaces of a tool that contact a material contain random, microscopic irregularities that are produced during the tool's manufacture and/or subsequent wear through use and abuse.  These irregularities which are formed randomly, are considered unique and can individualize or distinguish one tool from another.  Because these irregularities or individual characteristics are typically imparted onto the work piece, the comparative study of the imparted markings allow the tool to be individually associated or identified as having produced the mark.  The presence, observation and comparison of these random toolmarks on tools form the hypothetical propositions upon which the discipline of Toolmark Identification is based.

The most widely accepted method used in conducting a toolmark examination is a side-by-side, microscopic comparison of the markings on a questioned material item to known source marks imparted by a tool.

The examination process used in 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) to the analysis and comparison to the most specific characteristics (individual).

Summary of Examination Methodology

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.

A good starting point for any examiner looking for guidance in admissibility proceedings are the two documents listed below.

  1. Firearm/Toolmark Identification: Passing the Reliability Test Under Federal and State Evidentiary Standards, Grzybowski, R., Miller, J., Moran, B., Murdock, J., Nichols, R., Thompson, R., AFTE, Vol. 35, #2, Spring 2003, pg 209-241
  2. Firearm/Toolmark Identification- Meeting the Daubert Challenge, Grzybowski, R., Murdock, J., AFTE, Vol. 30, #1, Winter 1998, pg 3-14.


The Five Prongs of Daubert

1.  Testability of the Scientific Principle

  • Scientific testing is a procedure for critical evaluation of a scientific methodology.
  • The methods applied to the microscopic comparison of toolmarks have been tested and re-tested over the course of the discipline's long history.
  • The following link provides a list of works that have tested the legitimacy of the methodologies used in the field.

Testability of the Scientific Principle

2.  Known or Potential Error Rate

  • Error rate is the frequency at which one deviates from a correct standard.
  • Errors can occur through individual oversight or as a result of the deviation from a particular method.  The forensic firearm and toolmark community participates in validity and proficiency testing from which error rates can be calculated.
  • The following link provides documents that outline the proficiency testing program and published error rates.

Error Rate Documents

3.  Peer Review and Publication

4.  Maintenance of Standards and Controls

The establishment and maintenance of operational guidelines for conducting analytical testing.

Firearm and Toolmark Identification has well-established controls and procedures.  Representative documents that detail these protocols include the AFTE Technical Procedures Manual, AFTE Theory of Identification,  AFTE Glossary, AFTE Training Manual, SWGGUN Guidelines, and In-house Protocols.  Additionally, the following selected citations are references that address issues in maintaining standards of quality.

These controls and procedures are continually subjected to rigorous review by internal quality assurance and external accreditation/standardization bodies. Some of these general accreditation and standardization bodies are listed below.

Laboratory Accreditation Bodies

Standardization Bodies

5.  General Acceptance in a Particular Scientific Community

General acceptance is the approval by a particular authoritative body of a technique or methodology.

Firearm and Toolmark Identification is well-grounded in the scientific method and has been generally accepted by the forensic science community for decades. Firearm and toolmark identification courses have been and continue to be taught in forensic science programs around the world. Funding of scientific research in the area of firearm and toolmark identification has been granted to researchers outside the firearm and toolmark community.

Academic Programs

Grant Programs (Examples)

The following is a list of some of the case citations that relate to the admissibility of Firearm and Toolmark Evidence in the United States and Canada. While some of the issues expressed in these cases regarding the qualifications of the individual examiner and the application of the scientific procedures may have merit, the scientific principles and the basis for which opinions are expressed in the field of Firearm and Toolmark Identification remain well founded.

Case Citations

The following are a list of some articles that relate to the field of Firearm and Toolmark Examination.


The following link provides a list of articles from legal, academic and other sources that have expressed support or concerns regarding the admissibility of Firearm and Toolmark evidence.

Supporting and Opposing Viewpoints