Most people now know that the fired casing can also be matched to the firearm. This is useful when dealing with any incident where casings have been ejected - an automatic, pump action, lever action, or even a revolver or single shot where the firearm was reloaded. The information on a casing is also valuable with unfired ammunition, such as when someone has cycled rounds through the firearm, perhaps from unloading and later reloading it. There are a variety of marks on the case that may be used to tie it to a firearm:
- Breech face marks, which are impressed onto the surface of the case head and primer during the firing of a gun
- firing pin marks, impressed onto the primer when the firing pin impacts it
- extractor marks, which are made by the extractor as it grabs the rim and removes the case from the chamber
- ejector marks, which are applied to the case head during the ejection cycle as the case head impacts the ejector and is knocked clear of the action
- magazine lip marks, from the edges of a magazine sliding across the forward edges of the case as it is inserted and removed from the magazine.
An often overlooked piece of information that can be gleaned from fired bullets is known as General Rifling Characteristics. Manufacturers use the same rifling characteristics as they make hundreds or thousands of any single model of firearm. An examiner, confronted with only a recovered bullet, can measure the widths of the lands and grooves, count the number of them, and determine the direction of rifling twist. Then, the examiner may refer to a collection of data, known as GRC. These collections, once printed and now available in computer formats (indeed, the ATF/FBI NIBIN provides access to such a collection through its database) provides lists of firearms with similar characteristics. Depending on the accuracy of the measurements, the examiner may be able to filter the list down to a very few, or even single, model of firearm. This provides strong investigative leads - I do not need to consider every 9mm Luger pistol if the GRC indicates it could only have originated from a very few models.
Occasionally, distance may be determined. In a shooting, the victim's shirt is found to have a bullet hole. Lab techniques permit the examiner to test it for the presence of various particulate matter - unburnt powder, burnt powder smoke residue, and primer elements. Using lab techniques, the examiner may visualize a pattern of these particles. Then, using the firearm involved in the case, the criminalist may conduct a series of test shots, over a variety of distances. Here, it is important for the examiner to know and thus use the same type ammunition as was involved before. The test fired target will then be subjected to the same lab tests to permit visualization, and a comparison against the shooting evidence permit an estimation of the distance involved in the shooting. Specific? No. But it does provide a good, generalized distance to be taken into consideration, especially if the question is between close range and moderate range shots. These tests are usually limited to about fifteen feet total range; the particulates do not carry further, and may not carry that far.
Shotgun permit a similar analysis. The combination of shotgun and shotshell load will produce a pattern that will expand over distance. By documenting the evidence pattern first, and then conducting a series of test shots with the evidence shotgun and the same type ammunition, the firearms examiner will be able to establish the pattern of the shotgun over a variety of distances, and then compare this to the evidence, to come up with an estimated range.
A common practice is to document the trajectories involved in a shooting. Most shootings occur within fifteen meters; at these ranges, ballistic curves do not have great significance. Thus the trajectory can be treated as a straight line. When provided with multiple points, the investigator can track the flight of individual shots, and plot them back to a potential shooting position. This permits the reconstruction of a scene, tracking especially the actions of the shooter.
In a recent case, victims claimed that during a road rage incident another driver had pulled up to their car, fired into the driver's window, the bullet missing the driver and skinning the passenger's forehead. The victims drove home, two counties away. The injured party would not cooperate with the investigation; no documentation was ever made of the wound. Upon returning to the county of jurisdiction, patrol officers did an excellent job of photographing the vehicle and the damage to the driver's window, plus the lack of any of the damage to the vehicle. Two days later the victim driver returned, to show investigators a bullet found in the passenger door map pocket. This was also well documented. A suspect had been developed from their statements; the suspect owned two rifles, which were turned over to investigators, plus showed them an empty box for a revolver, which was stated to have been lost some time previously. The rifles were submitted to a local crime lab; they could not be identified to the bullet. No GRC was conducted. During defense reconstruction, a series of tests were conducted with both rifles and revolvers to determine the behavior of a bullet under the circumstances. In every case, the bullet passed through the first piece of window glass, passed through a witness board representing the forehead, and passed through the second window glass, with no deviation nor apparent loss of energy. The physical evidence would not support the projectile's appearance in the map pocket. Upon examination of the defense's forensic review, the prosecution offered the defendant a very minimal misdemeanor plea to avoid trial.
When looking at a case, do not overlook the total potential of firearms evidence. It may have much more to tell than is readily apparent. Having your team examine it in detail may bring to light aspects of the case which were previously overlooked.
