Evidence Without Related Firearm

A typical approach for the intercomparison of evidence cartridge cases or shotshell cases includes the following steps:

1. Determine that the evidence cartridge case or shotshell case does in fact have microscopic marks of value for comparison purposes and is suitable for comparison.
2. Mount the best evidence cartridge case or shotshell case on the right stage with the base oriented upwards.
3. Orient the light source to illuminate the marked area of the base obliquely.
4. At low magnification (10x-20x), carefully examine the base and primer area. Rotate the cartridge case or shotshell case slowly around its long axis while seeking the best areas of individual characteristics on the primer and/or the surrounding base area. (Higher magnifications can be used later to verify the correspondence of finer impressed or striated detail.) When the best area is found, let the cartridge case or shotshell case on the right stage remain in that position.
5. Adjust the light source for the left stage at the same oblique lighting angle as the light source for the right stage.
6. Mount another evidence cartridge case or shotshell case on the left stage and rotate it on its long axis until it is in the same orientation as the evidence cartridge case or shotshell case on the right stage.
7. Confirm that the evidence cartridge cases or shotshell cases bear consistent class characteristics with regard to location, size, and type of marks. If the class characteristics are the same, the examination should proceed.
8. Manipulate both microscope stages so as to align any corresponding microscopic impressions (e.g., breech face marks) or striated marks (e.g., primer shearing) that may be present on the primers and/or the base areas of the evidence cartridge cases or shotshell cases. If corresponding microscopic detail is present for comparison purposes, the examiner can then conclude if there is sufficient agreement in quality and quantity of the individual characteristics to substantiate an identification. The corresponding areas should be indexed using a permanent felt tip marker for possible future reference.
9. The area of best agreement should be documented, preferably by digital or conventional photography, or by sketching or narrative according to laboratory protocol. Images should be marked with the examiners initials, case identifier, degree of magnification, evidence and test item numbers, and a description of what is portrayed.
10. If the evidence cartridge cases or shotshell cases are damaged or deformed in some way, it may be necessary to use other areas of evidence and test items for comparison purposes.
11. To further compare the evidence cartridge cases or shotshell cases, their firing pin impressions should also be examined and compared. In order to better illuminate the interior of firing pin impressions and to compensate for depth-of-field limitations of the microscope, it may be necessary to tilt the evidence specimens relative to their respective light sources. In addition, it will be necessary to also incrementally rotate both cartridge cases around their long axes in order to fully appreciate the microscopic detail within the firing pin impressions. In some cases it can be productive to cast firing pin impression surfaces in order to detail subtle marks which are not readily visible except by casting. Mikrosil is one example of casting material especially valuable for this purpose.
12. Again, if corresponding microscopic detail is present for comparison purposes, the examiner can then conclude whether or not there is sufficient agreement in quality and quantity of the individual characteristics to substantiate identification. The corresponding areas should be indexed using a permanent felt tip marker for possible future reference.
13. In addition, the extractor marks, ejector marks, chamber marks, magazine lip marks, anvil marks, ejection port marks, and other possible mechanism marks should be compared. Comparison of these marks may require a number of reorientations on the microscope stages in order to best view the areas of interest on the evidence items.
14. Yet again, if corresponding microscopic detail is present for comparison purposes, the examiner can then conclude whether or not there is sufficient agreement in quality and quantity of the individual characteristics to substantiate an identification. The corresponding areas should be indexed using a permanent felt tip marker for possible future reference.

When a large number of evidence items are recovered at a crime scene, they may bear various degrees of damage and mutilation. Difficulties can also occur when good-quality marks are reproduced incompletely on each fired cartridge case or shotshell case. The marks can be more than sufficient for an identification, but different portions of them must be used for each pair of evidence items compared. This establishes a valid linkage between the evidence items. These results are as valid as any other identification.

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