THE FOLLOWING ARTICLE HAS APPEARED IN THE RECORD ON February 5, 2003

The importance of ballistic evidence Published Feb 5th


If and when the two DC area snipers are convicted it will be in large part thanks to scientific ballistic evidence. The same can be said about most murderers who resorted to the use of a firearm and who are standing trial, the reason being that it is among the strongest evidence.

Since guns are involved in about one out of three murders in Canada (total of 200 versus 14,000 in the United States) murder trials all across the country focus on this type of highly specialized scientific evidence. In the process they provide both jurors and spectators with a textbook lesson on ballistics.

Like most prosecutors I learned all about ballistics from a handful of experts who spent a lifetime testifying in court. Whenever one of my murder trials involved the use of a firearm I was required to prove that the fatal bullet was fired from the accused's gun. To do so, I regularly depended on ballistic experts who also worked for the Department of Justice and whose job focused on the functioning of firearms. .



While ballistics began as the study of the flight paths of projectiles, it now includes the study of practically everything that has to do with firearms including what happens inside of the firearm, during the bullet's flight and finally, when the projectile strikes the target.



As long as a suspect bullet has not been badly deformed or broken up upon impact with a target it is potentially capable of providing some of the best physical evidence linking a crime to an accused. Between a ballistic expert and pathologist it is often possible to determine not only the identification of the weapon from which the bullet was fired but also the distance and angle of entry traveled by the bullet.



An expert's cursory examination of the interior of the barrel of a firearm discovered near a crime can establish whether it was fired recently and if it was fired close to the victim.



Linking bullet to accused's gun

How is it possible to link a bullet from a victim's body to a specific gun? The science of ballistics is first and foremost based on the recognition that, much like fingerprints, no two guns are alike. With the exception of a shotgun, whenever a gun is fired, microscopic characteristics or imperfections are transferred through the gun barrel to the bullet and cartridge casings in the form of scrapes, grooves, scratches, marks or indentations.

Why the imperfections and why specifically the barrel? In order to stabilize the flight of a bullet and to improve the accuracy of the gun, gun manufacturers cut into each gun's barrel one or more lands spiral grooves ("rifling" a barrel) down the length of the barrel's inside, sometimes referred to as ridges.

Whenever a weapon is fired the pressure causes the bullet to expand slightly to fill the grooves, which makes the bullet spin, improving the accuracy of the gun. While the bullet spins and accelerates, the barrel of the gun cuts small grooves into the bullet that are unique to the gun from which it was fired.

Shotguns leave no markings because the barrels are smooth, without lands and grooves.

One of the most frustrating things for investigators and prosecutors is having a suspect and a bullet that is so mutilated that it is impossible to obtain identifiable markings. Occasionally markings on bullets or shell casings fired from older or poorly manufactured weapons are also unidentifiable. Soft bullets, such as those made completely of lead, sometimes flatten, which eliminates any detection of markings. Some weapons leave lighter impressions, making the markings difficult to see.




Shell casings provide important information



Even a shell casing, which is the rear cylinder of the cartridge that holds that holds the gunpowder, is capable of providing important information. First and foremost it indicates the manufacturer's brand and the caliber of the bullet. In many instances a ballistic expert is also able to link a shell casing to a specific gun. This is because an impression is left on the rear of a shell casing when the firing pin strikes it, which is as distinct to each weapon as the lands and grooves in a gun's barrel. Furthermore, unique marks are also made on the casings at the time they are ejected out of the gun.



How the analysis is made



Any round fired from a weapon can leave up to three types of clues for ballistic experts to analyze. They include markings on the sides of the bullet, marks on the back of the shell casing and marks on the side of the shell casing. A trained ballistic examiner will compare the evidence under a microscope and determine whether the objects can be matched to the weapon.





One common method involves the taking of bullets, fragments, or empty casings and test firing samples from the recovered gun. The process is photographed with digital cameras and converted into computer images. A computer then compares the characteristics of a bullet or casing with thousands of other stored images.

Computed tomography (CT) scanning is now being used to determine the caliber of a bullet when it is not possible to remove the bullet from the body. This sort of imaging produces cross-sectional images or "slices" of anatomy, like the slices in a loaf of bread. As the patient passes through the CT imaging system, a source of x rays rotates around the inside of the circular opening.

Linking the weapon to the suspect



Inasmuch as ballistic evidence is capable of linking a fatal bullet and shell casing to a gun there still remains an equally important hurdle to cross in any murder trial. The weapon itself must be linked to the accused. This is where both police and civil evidence is important. Very few suspects are found standing over the body of their victims, holding a gun with a wisp of gun smoke coming from the barrel. Finding the deadly firearm in the possession of the suspect, whether on his body, in his vehicle or residence or buried on his property are only a few of dozens of different scenarios. Locating the person who sold or gave the weapon to the suspect is equally important. Other methods include fingerprints, DNA evidence, and finally, the suspect's confession.

Ballistic experts are only one of many different forensic investigators prosecutors depend on. In future columns I hope to write about Forensic artists, Forensic chemists and Forensic pathologists.