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. Athena Research & Consulting LLC |
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. Athena Research & Consulting LLC |
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Forensic Firearms and Ballistics |
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Ballistic Matching and Toolmark Analysis |
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Overview Often referred to as ballistic fingerprinting, this is the science of matching manufacturing imperfections on the firearm, to the marks they leave on the bullet and cartridge case. The key tool used is the comparison microscope - a large, heavy, and expensive instrument which is produced by linking two microscopes with an optical bridge, see Figure 5. There are a number of points where the ammunition and firearm components come into contact, these are: * Bullet in the barrel * Rear of the cartridge case against the breech * Firing pin indentation in the primer * Extractor around the case periphery, or rim * Ejector on the rear of the case * Loaded chamber indicator pin * Magazine lips on the side of the cartridge case The first two of these regularly yield positive identifications, and firing pin indentations can be a useful ID source also. The last four have small areas of contact and, consequently, yield positive identifications less frequently. It should be noted that the suspect firearm must be available for test firing to enable these comparisons to be performed. |
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Bullet Matching When a bullet goes through a barrel it is engraved by the rifling, and each set of rifling grooves has unique machining marks, which in turn leave a unique and identifiable imprint on the bullet itself. The resulting scratches on the bullet surface are known as striations, and they can be used for comparison and identification purposes - a similar concept to the human fingerprint. An example of a comparison of two fired bullets is illustrated in Figure 1. A comparison would normally be made at a higher magnification, but the rifling grooves are visible in this image. Click on the picture for a larger view.
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Figure 1 - Two fired bullets under a comparison microscope |
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Breech Face Matching The breech face is the part of the firearm that closes up behind the round of ammunition. It almost always has machining marks. These machining marks are unique, and when the gun is fired the rear of the cartridge case is forced against the breech area under considerable pressure. The machining mark imprint is left on the base of the relatively soft brass cartridge case. Figure 2 shows a comparison microscope view of the bases of two fired cartridges. Note that the letters stamped around the periphery of the case do not need to match because they the cases are randomly orientated when fired. The breech face machining marks can be seen running horizontally in the picture. Click on the image for a larger view.
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Figure 2 - Two fired cases under a comparison microscope |
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Firing Pin Indentation Matching When a cartridge is fired the firing pin in the firearm must strike the primer located in the rear center of the cartridge. This leaves a substantial impression. Like all manufactured components, the firing pin may have unique markings from the production process. The images below show typical comparisons of firing pin indentations in primers.
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Figure 3 - Two firing pin indentations under the comparison microscope |
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Figure 4 - Two very distinctive firing pin indentations compared |
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Copyright ã 2001 ARC |
