Firearms Without Related Evidence
Test cartridge cases or shotshell cases should be intercompared to determine if the firearm in question is reliably reproducing microscopic detail of value for comparison and identification with a single firearm. The results of this intercomparison will provide an examiner with a basis for assessing the potential of the firearm for reproducing microscopic marks. It also provides a preview of the order of difficulty of the comparisons with actual evidence items.
The following intercomparison procedure applies to cases:
- Mount one of the test cartridge cases or shotshell cases on the right stage with the base oriented upwards.
- Orient the light source to illuminate the marked area of the base obliquely.
- At low magnification (10x-20x), carefully examine the base and primer area. Rotate the cartridge case or shotshell case slowly around its long axis to find the best areas of individual characteristics on the primer and/or the surrounding base area. Once the best area is located, the case should remain in that position on the right stage.
- Mount another test 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 case on the right stage.
- Adjust the light source for the left stage at the same oblique lighting angle as the light source for the right stage.
- Manipulate both microscope stages so as to align any corresponding microscopic impressions or striated marks that may be present on the primers and/or the base areas. 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.
- To further evaluate the potential value of test cartridge cases and 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 both test specimens relative to their respective light sources. It will be necessary to incrementally rotate both cartridge cases around their long axes in order to fully evaluate 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.
- If corresponding microscopic detail is present for comparison purposes on the firing pin impressions, 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.
- In addition, the extractor marks, ejector marks, chamber marks, magazine lip marks, anvil marks, ejection port marks, and other possible mechanism marks should be evaluated. Comparison of these marks may require a number of reorientations on the microscope stages in order to best view the areas of interest.
- As with the other comparisons, if corresponding microscopic detail is present, the examiner can then conclude whether or not there is sufficient agreement in quality and quantity of the individual characteristics for an identification of the test cartridge cases or shotshell cases as having both been fired in the same firearm or cycled through the action of the same firearm, depending on the type of mark being identified. The corresponding areas should be indexed using a permanent felt tip marker for possible future reference.
At this point, the examiner has
- observed the types of microscopic marks being produced by an evidence firearm,
- assessed the quality of these marks for comparison purposes,
- noted the peculiarities of the particular firearm,
- documented the pertinent observations.
Additional Online Courses
- What Every First Responding Officer Should Know About DNA Evidence
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- Law 101: Legal Guide for the Forensic Expert
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