To determine if a bullet that has been tested is the same bullet gathered from a crime scene, forensic experts typically employ several methods and techniques:
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Ballistics Analysis: This is the primary method for identifying bullets. Each firearm leaves unique markings on bullets as they are fired. When a bullet is fired, it comes into contact with the barrel of the gun, and the combination of the barrel's unique characteristics and any wear and tear results in distinct striations and impressions on the bullet surface.
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Comparison Microscopy: In the forensic laboratory, investigators use a comparison microscope, which allows them to view two bullets side-by-side simultaneously. The expert can examine the individual characteristics of the striations and other markings to see if they match.
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Firing Pin Impressions and Land and Groove Marks: Aside from striations, forensic experts will also look at other unique features like firing pin impressions and the specific patterns created by the lands (raised parts) and grooves (depressed parts) inside the barrel of the firearm.
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Microscopic Analysis: Advanced imaging techniques, such as scanning electron microscopy (SEM) or 3D imaging, can be used to analyze the bullet’s surface details at a microscopic level, making it easier to identify unique characteristics.
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Physical Matching: In cases where the bullet has fragmented or been damaged, forensic experts may sometimes be able to physically match the pieces of a bullet to a whole bullet, based on the fit of the broken edges.
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Trace Evidence: Additional testing may also involve assessing trace evidence, such as the presence of gunshot residue or other particulates adhering to the bullet, which may link it to a crime scene or a specific firearm.
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Documentation and Chain of Custody: It's critical that the evidence from the crime scene is collected, preserved, and documented properly to ensure that it can be reliably linked to the allegations. A clear chain of custody should be maintained to confirm that the bullet tested is indeed the one collected from the scene.
When all these methods converge on the same conclusion—that the bullet from the crime scene matches the test bullet—it strengthens the case that they are the same.