NIJ, Firearm's Examiner Course, Module 11, retrieved April 25 2020 ,https://firearms-examiner.training.nij.gov/
© Sara Hancock, April 2020

Firearm Identification: firearms related specialty that is a subdivision of toolmark identification.
A fired bullet engages the rifling in the interior of the barrel which leaves unique microscopic characteristics that result in the process of manufacturing. Striations are contour variations on the surface of an object caused by force and motion. Subclass Characteristics can also be present on the fired bullet surface.

Physical Characteristics

  • Class characteristics: measurable features that indicate a restricted group source. The general rifling characteristics of the barrel are the class characteristics of interest.
  • Individual characteristics: marks that are random imperfections or irregularities. They are produced incidental to manufacture and/or caused by use, corrosion, or damage.
  • Subclass characteristics: discernible surface features of an object that are more restrictive than class characteristics. They are produced incidental to manufacture, relate to a smaller group source, and identifiable within a time frame.

Pattern Identification
Pattern matching is the process of determining whether or not the striated toolmarks on 2 objects correspond. This is a subjective process with the goal of pattern identification. Pattern matching is the standard method for firearms identification.

Consecutive Matching Striae
quantitative method of describing an observed pattern match. Using CMS is a means of the best known non-match described and defined by the AFTE Theory of Identification. These criteria are of increasing importance in consideration of: the expectation of sophisticated jurors, need for more objective identification criteria, changing environment after Daubert, potential for increased credibility for examiners in the courtroom.

AFTE Theory of Identification

  1. Theory of identification enables opinions of common origin to be made when unique surface contours of two toolmarks are in significant agreement.
  2. The sufficient agreement is related to the signification duplication of random toolmarks as evidence by the correspondence of a pattern or combination of patterns of surface contours. If sufficient agreement exists between 2 toolmarks mean 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.
  3. The interpretation of individualization/identification is subjective in nature founded on scientific principles and based on training and experience.

Sufficient Agreement and Best Known Nonmatch
The examiner will gain experience during initial training period about degrees of correspondence. The examiner should also have a second qualified examiner verify the scientific findings. The sufficient agreement is the product of the individual examiners training, skills, and experience in recognizing corresponding patterns of matching striations and within impressed toolmarks, determine the best known non-match in their personal experience, and comparing striated and impressed toolmarks.

Range of Conclusions:

  • Identification
  • Inconclusive
  • Elimination
  • Unsuitable for comparison

Identification: agreement of a combination of individual and all discernible class characteristics where the extend of agreement exceed 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:

  • Some agreement of individual and class characteristics but insufficient for identification
  • Agreement of class characteristics without agreement or disagreement of individual characteristics due to an absence, insufficiency, or lack or reproducibility,
  • Agreement of all class characteristics but disagreement of individual characteristics, but insufficient for elimination.

Elimination:
A significant disagreement of class characteristics and/or individual characteristics. An elimination based on individual characteristics require a detailed knowledge of history and treatment of the firearm. Some laboratories do not allow this.

Unsuitable:
Appropriate for fired bullet fragments that do not bear microscopic marks of value for comparison purposes. These may be: fired bullet fragments, jacket fragments, lead bullet cored, lead fragments, other portions of fragmented, mutilated, or damaged bullets, or metallic fragments that cannot be identification as having been a part of a fired bullet.

Bullet Examination
The foundation for this technique is based on:

  • The rifling of the barrel of a firearm bears unique microscopic characteristics due to manufacturing processes, us, and abuse.
  • These characteristics mark the bearing surface of bullets when fired through the barrel.
  • These individual characteristics are reproducible and identifiable with a particular firearm.

Microscopic Procedures:

  • Review laboratory protocols
  • Complete administrative requirements
    -Chain of custody, marking of evidence, laboratory case identifiers, investigative file identifier, examiner identity, quality assurance.
  • Follow lab safety protocols
  • Determine presence of trace evidence and follow protocol for collection (includes: paint, glass, blood, bone, soft tissue, hair, fibers, wood, metal smears, masonry, and stone).
  • Determine if latent fingerprint examination should be performed prior to examination.
  • Determine if other examinations should be performed.

Recovered Firearm without Related Evidence:
Firearms should be test fired and bullets recovered for the following reasons:

  • To determine if the firearm reliably reproduces microscopic marks on test fired bullets that are suitable for comparison purposes
  • To allow the examiner to assess the quality of marks produced by the firearm
  • To retain physical samples of bullets fired from all firearms entering the lab.
    Retain bullets for comparison with new evidence submissions and enter into digital image reference files.

Comparison Process:

  1. Recover test fired bullets
  2. Mark test bullets and indicated sequence of recovery
  3. Use stereomicroscope to determine presence/quality of microscopic marks
  4. Place on fired bullet on right stage of comparison microscope and attach the nose of the bullet to the mount.
  5. Adjust the lighting from the rear of the microscope to provide oblique illumination over the bearing surface of the bullet.
  6. Examine entire bearing surface at low magnification and find best area of individual characteristics.
  7. Place the second test bullet on the left stage with the nose in the same direction as the right stage.
  8. Rotate bullet on left stage to same position as right stage and set the high magnification.
  9. If area is found, align impressions and examine/compare until conclusion can be drawn.

Examinations should be made with bullets in phase. Agreement is sufficient when it exceeds the best level of agreement demonstrated between bullet striations known to have been produced by different barrels. If an identification cannot be made between the test bullets, then more test bullets should be fired and compared. If an identification can still not be made then the examiner may reach the conclusion that the barrel may not consistently produce sufficient marks. If an identification can be made, the area of best agreement should be indexed with a permanent marker for later use in comparisons.

Recovered Firearm with Related Evidence
1.Mark recovered evidence

  1. Use stereomicroscope to determine that recovered bullet has marks of value.
  2. Use comparison process described off of recovered firearm without related evidence.
  3. Place best test bullet on right stage.
  4. Place recovered evidence on left stage.
  5. Adjust lighting equally to provide oblique illumination.
  6. Rotate test bullet, placing the previously marked index area in the center of viewing area.
  7. Align land impression on the recovered evidence bullet with the indexed area on the test bullet. Confirm whether or not the widths of the observed impression are the same for both bullets.
  8. Rotate the recovered evidence bullet to search for individual characteristics similar to those found on previously indexed area, if found, compare all undamaged bearing surfaces of the recovered evidence.
  9. Proceed with additional exams if the recovered bullet cannot be indexed, identified, or eliminated.

Potential Elimination:

  • If the widths (8) are significantly different then the recovered evidence bullet was not fired from the recovered firearm. If significant damaged is present, follow lab protocols.

Potential Identification:

  • If the recovered bullet is damaged/deformed in the area previously indexed on the test bullet, it may be necessary to use other areas.
  • If there is sufficient agreement for an identification, the recovered evidence bullet should be indexed at the same area as the test bullet
  • If the recovered bullet is missing the area indexed on the test bullet, the best area for comparison on the recovered evidence bullet should be indexed with a different color. The test bullet must be indexed again at area of best agreement.
  • Document area of agreement according to lab protocol.

Inconclusive reasons:

  • Recovered evidence bullet and the test bullet were fired from different firearms
  • Damage occurred to recovered evidence bullet that caused distortion, deformation, or eradication of microscopic detail.
  • The type of test ammunition was different from the recovered evidence bullet.
  • The barrel contains metal deposits
  • Corroded deposits were dislodged from the barrel during firing causing changes
  • The firearm was damaged during the time interval between firing recovered evidence and test bullet.
  • Some or all chambers in a revolver are misaligned, causing differences in the microscopic marks found on bullets fired from different chambers.

Before an inconclusive result:

  • Remount both bullets and reexamine
  • Clean barrel of evidence firearm and obtain new test fires
  • Use magnesium smoke to enhance detail.

Evidence without Related Firearm:

  1. Mark recovered evidence bullet
  2. Use stereomicroscope to determine that recovered bullet have marks of value.
  3. Place best bullet on right stage of comparison.
  4. Adjust lighting to provide oblique illumination.
  5. Examine entire surface using low magnification and leave in best position
  6. Place second evidence bullet on left stage
  7. Adjust lighting to provide oblique illumination.
  8. Align land/groove impression of 2 bullets to determine if widths are same.
    Follow protocols relating to Recovered with Related Evidence
    ...
  9. The above process should be repeated with all evidence bullets.

Case Linkages: May be determined by examining fired bullets recovered from the bodies of victims or from crime scenes. Bullets are compared to all previous bullets in the appropriate caliber. IBIS helps with these comparisons now.

Casting
Bullet surface replication can arise due to NIBIN-related high-probability associations made in remote lab that must be physically examined using a comparison microscope, routine liaison with examiners from other jurisdictions concerning possibly related cases, and information provided to investigators by an informant indicating possible case linkages between cases from different jurisdictions.

Administrative Obstacles: chain of custody concerns, policies relating to shipment/transport of evidence, budgetary constraints regarding long distance travel.

Procedure:
Casting material for toolmarks include Mikrosil, Coe-Flex, and Dip-Pak among others. They can accurately replicate the microscopic details on the surface of fired bullets. The procedure is:

  1. Mix components of casting material until the mixture begins to harden.
  2. Apply mixture to the surface of the fired bullet and allow to completely harden.
  3. Remove after hardened.