Recovered Firearm With
Two test bullets with land and groove
impressions aligned, showing an
area of correspondence
The comparison of recovered evidence bullet(s) with test bullets from a recovered firearm should be performed according to laboratory protocol.
A sample procedure follows:
- Mark the recovered evidence bullet.
- Use a stereomicroscope to determine that the recovered evidence bullet has microscopic marks of value and is suitable for comparison.
- Use the comparison process described previously to confirm that the test bullets from the recovered firearm can be identified with each other.
- Place the best test bullet on the right stage of the comparison microscope. Attach the nose of the bullet to the mount.
- Place the recovered evidence bullet on the left stage of the comparison microscope. Attach the nose of the bullet to the mount.
- Adjust the lighting equally from the rear of the microscope to provide oblique illumination over the bearing surface of both bullets.
- Rotate the test bullet, placing the previously marked index area in the center of the viewing area.
- Align a land impression (or a groove 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.
- If the widths of the observed impressions are significantly different, it may be possible to conclude that the recovered evidence bullet was not fired from the recovered firearm. This concludes the examination of this recovered evidence bullet.
Note: Such a difference could be due to deformed, damaged, or mutilated evidence bullets. If significant damage is not present, laboratory protocol for elimination should be followed (many laboratories require a minimum difference of 0.005” for elimination).
- If the widths of the observed impressions are not significantly different, the examination should proceed.
- Rotate the recovered evidence bullet to search for individual characteristics similar to those found on the previously indexed area of the test bullet. If these are found, compare all undamaged bearing surfaces of the recovered evidence bullet with the corresponding areas on the test bullet.
- If the recovered evidence bullet is damaged or deformed in the area previously indexed on the test bullet, it may be necessary to use other areas of the test bullet to compare to the undamaged areas of the recovered evidence bullet.
- If there is sufficient agreement for an identification, the recovered evidence bullet should be indexed at the same area as the test bullet, even if this is a damaged area.
- If the recovered evidence 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 the area of agreement and with the same color as used on the recovered evidence bullet.
- Document the area of agreement according to laboratory protocol.
- Proceed with additional examinations if the recovered evidence bullet cannot be indexed, identified, or eliminated as having been fired from the recovered firearm. These additional examinations are performed to determine the conditions or circumstances that may have caused the inconclusive result.
Reasons for an inconclusive result may include the following:
Before deciding upon an inconclusive result, the following potential remedies should be explored:
- The recovered evidence bullet and the test bullet were fired from different firearms.
- Damage occurred to the 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 (leading).
- Corroded deposits were dislodged from the barrel during firing, causing significant changes in the barrel.
- The firearm was damaged during the time interval between firing the recovered evidence bullet and the test bullet.
- Some or all chambers in a revolver are misaligned, causing differences in the microscopic marks found on bullets fired from different chambers.
- Remount both bullets on their respective bullet mounts and reexamine.
- Clean the barrel of the evidence firearm and obtain new test bullets. Bullets tested before and after cleaning must be retained.
- Use magnesium smoke to enhance detail (smoking).