Characteristic parallel linear striations on the percussion cap around the firing pin impression were observed on a fired cartridge case suspected to have been fired through a self-loading rifle. The firing pin impression also bore similar striations, both at the bottom and on the sides.

On examining the test cartridges fired through the suspected rifle, and comparing them with the questioned cartridge, the following observations were made and the results interpreted:

• a) The firing pin and the ejector marks on the questioned cartridge and the test cartridges were found to be similar in their individual characteristics indicating a common source of origin.
• b) The characteristic linear striations present on the questioned cartridges were NOT present on the test cartridges indicating non-linkage of the questioned cartridge with the suspected firearm.

The aforesaid examination and comparison thus lead to obvious contradictory conclusions. To arrive at the correct conclusion, the source, which produced the linear striations on the questioned cartridge, was thoroughly investigated. The striations inside the firing pin indentation and on the rest of the percussion cap, when examined simultaneously under the microscope revealed continuity from one end to the other, even when they were passing through the firing pin indentation. (FIG 1)

FIG1

The aforesaid observations could be explained in the following way to decide the issue whether the striations on the questioned cartridge were produced as a result of firing or otherwise:

• a) The continuous striations could not occur during firing because the gap between the periphery of the firing pin and its hole would exhibits discontinuity of lines on the fired cartridge between the breech striations and the firing pin striations, even if they happen to be in the same straight line.
• b) Presence of striations over the whole surface of the indentation including slope clearly indicates they were neither breech scrape or firing pin scrape.
• c) Tampering of the cartridge after firing by filing or rubbing against a rough surface would also not produce such continuous striations, especially inside the indentation because the firing pin indentation was at a lower level.

Thus the possibility of producing such continuous striations, during or after firing, could be eliminated. It was evident that the striations existed on the base of the live cartridge. They could have come either at the time of manufacture, handling or storage. Therefore, they could safely be ignored while considering the linkage of the questioned cartridge with the firearm.

The case is recorded so that other experts are not mislead by the presence or absence of such striations. A caution, therefore, is needed while evaluating continuous striations on the percussion cap, especially when they pass through the firing pin indentation. The possibility of their presence before firing should also be probed into while forming an opinion.

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Cartridge Identification by Chamber Marks without test firing
Unusual Chamber Marks
Test extractor marks under low pressure on Shotgun Cartridges
 
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