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Sherlock Holmes Forensic Series: Ballistics

Ballistics is a science that deals with the motion of projectiles. A forensic ballistics specialist like myself is a person who collects studies and analyzes evidence related to ammunition and firearms. When a bullet is found at a crime scene or removed at autopsy, I may be asked to determine where that bullet came from. In other words, to identify the firearm from which it was discharged.

Once a bullet leaves the gun barrel, trajectory comes into play. If bullet holes are found in floors, walls or objects at the scene of a crime, the trajectory or angle of the bullet’s path as well as probable distance may be determined. Lasers, strings or even probes can be used to visually mark the path of the bullet by positioning one or more of them through the holes and following it to a potential source. If bullet fragments and discharge casings are found, they can be collected as evidence to help reconstruct what took place during the shooting.

Let’s look inside a gun to see what happens when it’s fired. The barrel contains spiral grooves that cause a bullet to spin improving its stability and accuracy. This is called rifling. When it passes through the gun barrel, the bullet takes on a mirror image of the rifling called engraving. Engraving consists of major features of the barrel that are visible to the naked eye but also minor features like distinctive scratches and marks that can only be seen through a microscope. It is these characteristics which allow a firearm examiner to associate or exclude a recovered piece of ammunition that a firearm from which it was discharged.

In performing the lab work, an invaluable instrument known as the comparison microscope is used to analyze specimens side by side. It consists of two microscopes connected by an optical bridge resulting in a split view image allowing two separate objects to be compared simultaneously. So, when there’s a bullet or a cartridge case from a crime scene and a firearm has been recovered that may have been involved in the incident, we can fire a cartridge from this firearm, then optically compare it to the bullet or case found at the crime scene.

With the comparison microscope, we can see if striations on the two bullets match. If significant agreement exists between the two examples, a conclusion of common origin can be made, suggesting that a certain firearm was responsible. This is an excellent example of how forensic science has advanced since the time of Sherlock Holmes.

Sherlock Holmes Exhibit

The International Exhibition of Sherlock Holmes was developed by Exhibits Development Group and Geoffrey M. Curley + Associates in collaboration with the Conan Doyle Estate Limited, the Oregon Museum of Science and Industry, and the Museum of London.

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