The reconstruction of a shooting incident is the most challenging part of a firearms examiner's job. This is the culmination of all the scientific and criminal investigations relating to a particular incident wherein a theory for the shooting is postulated. Unless such a theory can be developed, the individual firearms examination results are often inadequate in and of themselves to explain the shooting circumstances to a jury in a criminal proceeding. Shooting reconstruction requires specialized knowledge and equipment beyond that of the typical firearms examiner. This means specialized training and significant field experience are necessary for a firearms examiner to be qualified to reconstruct shooting incidents.
There are special considerations involved in the reconstruction of shooting incidents. For example, examiners may want to estimate the shooter's position. As a result, there are special needs in terms of equipment. For starters, trajectory rods are inserted into bullet holes to determine bullet trajectories. An angle gauge is used to determine angles of impact into vertical surfaces. A laser attached to the end of a trajectory rod allows the trajectory rod to be "extended" out great distances. The first photograph shows the basic kit, which includes rods, angle gauge, tripod mount, and centering cones.
The second photograph shows a trajectory rod inserted into a bullet hole through a portion of a solid core door. The laser has been attached to the end of the rod and the angle gauge has been put in place. In this way the investigator is able to establish the paths of bullets at crime scenes and ultimately determine the probable position of the shooter. By taking angle measurements the investigator can create a scale diagram to assist the jury in understanding the circumstances of the shooting.
It is easy to determine vertical angles using an angle gauge, but what about the side-to-side angle that must also be measured? There will be both a vertical angle and a horizontal angle of impact, as shown in the diagram. Both of these angles need to be determined to create a scale diagram.
To measure a nonvertical angle, a protractor is required because the angle gauge only works in the vertical, or Y direction. Angles in the horizontal, or X, direction can only be measured using a protractor. Not just any old protractor will work, however. The protractor must be such that the base is the point of origin (that is, has a value of zero degrees). This requires what is known as a zero-base protractor. A zero-base protractor is shown in the photograph.
The zero-base protractor may be used to measure both angles in trajectory analysis. However, when measuring the vertical (Y) angle of a trajectory rod on an uneven surface, such as a car side panel, it is necessary to use a standard plane of reference, such as a plane perpendicular to the ground. This is particularly important when there are multiple bullet holes that require trajectory analysis. If there is no common or standard plane of reference, the angles recorded have no meaning. The method used to overcome this dilemma is to hold a plumb bob (conical
metal weight) on a string out some distance from the surface. The zero-base protractor is then held parallel to the plumb line that represents a perpendicular plane, as illustrated in the photograph.
When the bullet hole being examined is in a thin material, such as thin, aluminum siding, it is not possible to use a trajectory rod to determine the angles. This is because the trajectory rod must have two surfaces, one in front and one in back, with at least an inch of separation to rest on in order to demonstrate the bullet trajectory. When the bullet-damaged material is too thin, a laser protractor is used in a process that involves measuring the bullet hole and calculating the angle of impact. The examiner then sets this angle on the protractor, making the laser project the trajectory.
Additional equipment may be required to reconstruct a shooting incident, and this varies depending on the particular circumstances of the shooting. For instance, the firearms examiner will often need to test the angle at which it appears that a bullet struck a particular target (any inanimate object). To do this a fixture with a rotating target holder is used. This allows the angle of the gun barrel to the target to be set precisely. It is much simpler to rotate the target and keep the weapon fixed in position than it is to try to position the weapon at a particular angle.
Angle-of-impact determination fixture (Courtesy of the author)
The fixture shown in the photograph is made up by the examiner and illustrates the fact that not all the needed equipment is available commercially. Thus, the examiner must be somewhat mechanically inclined and have good problem-solving skills in addition to the academic and training requirements already cited.
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