U.S. flag

An official website of the United States government, Department of Justice.

Firearms Examiner Training

Case Linkage

Home  |  Glossary  |  Resources  |  Help  |  Contact Us  |  Course Map

Case Linkage

Forensic firearm laboratories maintain files of fired evidence bullets from unsolved cases. This facilitates the possibility that linkages can be made between shooting incident cases involving serial shooters. It is known that a small percentage of the criminal population commits a large percentage of crimes; this also applies to firearms-related crime. Case linkages may be determined by examining fired bullets recovered from the bodies of victims or from crime scenes. This includes bullets from cases in which a firearm has not been located or connected to the evidence collected at the crime scene.

Bullets submitted to the laboratory are microscopically compared to all previous bullets of the appropriate caliber retained in physical files. These files may be referred to as unsolved case files, open case files, reference fired specimen files, or open case ammunition files.

Over time, open case files grew large and became difficult to use in many laboratories. This process was simplified by advances in technology that allowed for digital searches of files to identify potential case linkages based on high probability associations. However, final confirmation of a linkage still requires comparison by an examiner using a comparison microscope.

These technological advances resulted in a system for accomplishing these computer-based comparisons or correlations - the Integrated Ballistics Identification System (IBIS).

Read more about Computer Based Technologies in Module 7.


Bullet Surface Replication

Occasionally, it may be necessary to physically compare bullets when one of the bullets is not available to the examiner.

This situation can arise due to

  • NIBIN-related high-probability associations (hits) made in a remote laboratory that must be physically examined using a comparison microscope,
  • routine liaison with examiners from other jurisdictions concerning possibly related cases,
  • information provided to investigators by an informant indicating possible case linkages between cases from different jurisdictions.

However, there may be administrative obstacles that can prevent or delay the physical comparison, such as

  • chain of custody concerns (integrity of evidence) on the part of investigative agencies and prosecutors offices,
  • policies relating to the shipment or transport of evidence to other laboratories or jurisdictions,
  • budgetary constraints regarding long distance travel to other jurisdictions.

The most practical and expeditious solution in these circumstances may be to use casting materials to replicate the microscopic surface of a fired bullet. The cast is then shipped to the examiner who will perform the comparison. This casting approach is recognized in the AFTE Procedural Manual . It is an extension of the rationale used in casting of other types of toolmarks.

It is important to bear in mind that a cast must be compared with another cast, not with the casted bullet itself; a cast is the reversal of the bullet surface.



Microscopic comparison of two casts of fired bullet surfaces
Microscopic comparison of two casts of fired bullet surfaces
National Institute of Justice (NIJ) (see reuse policy).

There are several recognized brands of appropriate casting material for toolmarks, including Mikrosil, Coe-Flex, and Dip-Pak, among others.

These can accurately replicate the microscopic detail on the surfaces of fired bullets. These products are available in a number of colors.

The procedure for casting the surface of a bullet is as follows:

  1. Mix the components of the casting material until the mixture begins to harden.
  2. Apply the mixture to the surface of the fired bullet and allowed to completely harden.
  3. Remove the cast when it has hardened.


Selected Bibliography

The Selected Bibliography is a list of the writings that have been used in the assemblage of the training program and is not a complete record of all the works and sources consulted. It is a compilation of the substance and range of readings and extensive experience of the subject matter experts.

  1. AFTE Criteria for Identification Committee. 1992. Theory of identification, range striae comparison reports and modified glossary definitions AFTE criteria for identification committee report. AFTE J 24 (2): 336-340.
  2. Biasotti, A. A. 1959. A statistical study of the individual characteristics of fired bullets. J of Forensic Sci 4 (1): 34-50.
  3. Biasotti, A., and J. Murdock. 1984. Criteria for identification in firearms and toolmark identification. AFTE J 16 (4): 16-24.
  4. Biasotti, A.A, and J.E. Murdock. 1997. Firearms and toolmark identification: Legal issues and scientific status. In Modern Scientific Evidence: The Law and Science of Expert Testimony , ed D.L. Faigman, D.H. Kay, M.J. Saks, and J. Sanders, 124 151. St Paul: West Publishing Co.
  5. Brackett, J. 1970. A study of idealized striated marks and their comparisons using models. J of Forensic Sci Soc 10 (1): 27-56.
  6. Daubert v. Merrell Dow Pharmaceuticals , Inc. 1993. 509 U.S. 579.
  7. Grove, C., G. Judd, and R. Horn. 1972. Examination of firing pin impressions by scanning electron microscopy. J of Forensic Sci 17 (4): 645-658.
  8. Hatcher, J. 1935. Textbook of Firearms Investigation, Identification and Evidence . Plantersville: Small-Arms Technical Publishing Co.
  9. Hatcher, J. 1947. Hatchers Notebook . Harrisburg: Military Service Publishing Co.
  10. Hatcher, J.S., F.J. Jury., and J. Weller. 1957. Firearms Investigation, Identification, and Evidence . Harrisburg: Stackpole Books.
  11. Heard, B. 1997. Handbook of Firearms and Ballistics - Examining and Interpreting Forensic Evidence . West Sussex: John Wiley & Sons, Inc.
  12. Mathews, J.H. 1962. Firearms Identification, Volume I . Springfield: Charles C. Thomas.
  13. Mathews, J.H. 1962. Firearms Identification, Volume II . Springfield: Charles C. Thomas.
  14. Mathews, J.H. 1973. Firearms Identification, Volume III . Springfield: Charles C. Thomas.
  15. Miller, J. 2000. An examination of two consecutively rifled barrels and a review of the literature. AFTE J 32 (3): 259-270.
  16. Miller, J. 2001. An examination of the application of the conservative criteria for identification of striated toolmarks using bullets fired from 10 consecutively rifled barrels. AFTE J . 33 (2): 125-132.
  17. Moran, B. 2001. The application of numerical criteria for identification in casework involving magazine marks and land impressions. AFTE J 33 (1): 41-46.
  18. Murdock, J. 1981. A general discussion of gun barrel individuality and an empirical assessment of the individuality of consecutively button rifled .22 caliber rifle barrels. AFTE J 13 (3): 84-111.
  19. Ogihara, Y., M. Kubota, M. Sanada, K. Fukuda, T. Uchiyama, and J. Hamby. 1983. Comparison of 5000 consecutively fired bullets and cartridge cases from a .45 caliber M1911A1 pistol. AFTE J 15 (3): 127-140.
  20. Shem, R., and P. Striupaitis. 1983. Comparison of 501 consecutively fired bullets and cartridge cases from a .25 caliber raven pistol. AFTE J 15 (3): 109-112.
  21. Tulleners, F., and J. Hamiel. 1999. Sub class characteristics of sequentially rifled 38 special S&W revolver barrels. AFTE J 31 (2): 117-122.
  22. Valdez, S. 1997. Bullet identification from HK USP polygonal barrels. AFTE J 29 (3): 307-309.
  23. Weber, P., and D. Scott. 2005. Applying firearm identification procedures in the analysis of percussion caps. AFTE J 37 (1): 34-44.
  24. Wright, D. 2003. Individuality and reproducibility of striae on plastic wad components fired from a sawed-off shotgun. AFTE J 35 (2): 161-166.

Back Forward