Firearms With Related Evidence

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A typical approach for the comparison of evidence and test cartridge cases or shotshell cases includes the following steps:

Determine that the evidence cartridge case or shotshell case does have microscopic marks of value for comparison purposes and is suitable for comparison.
Determine if the test cartridge cases or shotshell cases can or cannot be identified with each other.
Mount the best test cartridge case or shotshell case 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.
Adjust the light source for the left stage at the same oblique lighting angle as the light source for the right stage.
Mount an 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 test cartridge case or shotshell case on the right stage.
Confirm that the evidence and test cartridge cases or shotshell cases bear consistent class characteristics as far as location, size, and type of marks. If the class characteristics are the same, the examination should proceed.
Manipulate both microscope stages 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.
The area of best agreement should be documented according to laboratory protocol.
If the evidence cartridge case or shotshell case is damaged or deformed in some way, it may be necessary to use other areas of evidence and test items for comparison purposes.
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.
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.

If no identifications have been made to this point, further examinations should be made to ascertain what conditions or circumstances might have caused this result.

These could include the following:

The evidence cartridge case or shotshell case was fired in different firearm.
Damage occurred to the evidence cartridge case or shotshell case that caused distortion, deformation, or elimination of microscopic detail.
The test ammunition used was significantly different from the evidence, causing a difference in the way the test cartridge case or shotshell case was marked.
The evidence firearm was damaged between firing of the evidence and test cartridge case or shotshell case.

Potential remedies that may be used in certain circumstances to produce better microscopic detail on test cartridge cases and shotshell cases include

changing the brand and type of ammunition being used to obtain test specimens,
cleaning the evidence firearm and re-firing test cartridge cases or shotshell cases.

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