Ballistics in crime scene investigation

DEFINITION: Study of the motion, behaviors, effects, and impact signatures of projectiles.

SIGNIFICANCE: When projectiles—whether bullets, bombs, or missiles—are involved in crimes, ballistics experts play a vital role in the investigations. Forensic scientists trained in ballistics can identify the specific types of firearms used in crimes based on bullets, shell casings, and other evidence found at crime scenes. By comparing this information with weapons belonging to possible suspects, they can confirm individual weapons as those used in the crimes.

A ballistic body is any object used to exert force to make another object move or change in form, state, or direction. A bullet, for example, is a ballistic body when it is propelled by the sudden increase of pressure that takes place within a handgun or other firearm when the trigger is pulled and a discharge of explosive powder propels the bullet forward in a direction dictated by the barrel of the weapon. When the bullet exits the weapon, it is subject to the laws of ballistics. As the projectile reaches its target, its velocity and cause distinctive entry and exit wounds.

The science of firearms is divided into four components: internal ballistics, transition ballistics, external ballistics, and terminal ballistics. Internal ballistics is the study of the forces that cause the acceleration of ballistic bodies; in the case of a bullet fired from a gun, internal ballistics is concerned with the detonation of the bullet, its discharge from the chamber, and its pathway through the barrel. Transition, or intermediate, ballistics is the study of the immediate effects on ballistic bodies as they leave the barrels of weapons; this area of ballistics focuses on forces such as air pressure, gravity, and air density, which act collectively on projectiles as their initial acceleratory force is reduced.

External ballistics is the study of projectiles’ flight through the air. This includes the examination of changes in velocity and trajectory of ballistic bodies during the time they are in flight from weapons to targets. The last component of basic ballistics, terminal ballistics, is concerned with the impacts of projectiles on the objects with which they come in contact. This includes the effects of impacts on projectiles themselves and the ways in which bullets penetrate various surfaces (including human flesh).

Criminal Cases

Because the barrels of firearms are rifled (that is, they have raised and lowered spiral surfaces) to impart spin to bullets, distinctive marks (striations) are left on bullets as they swirl down the shafts of barrels after firing. The first recorded use of such marks as evidence in a criminal case took place in 1835. It was found that bullets fired from a weapon taken from the home of the primary had a distinctive ridge that was identical to the ridge seen on a bullet recovered from the scene of the crime. When confronted with this evidence during questioning, the suspect confessed to the crime. Nearly seventy years later, in 1902, attorney Oliver Wendell Holmes, Jr., introduced ballistics evidence in a court of law. In a case, Holmes had a local gunsmith test fire a weapon belonging to the suspect into a wad of cotton stuffing. Under magnification, the marks on the test-fired bullet were seen to match those on the bullet retrieved from the crime scene, and this evidence was presented to the jury.

Shortly thereafter, two ballistics experts of that time, Calvin Goddard and Charles Waite, began compiling a database of information on all known gun manufacturers and on specific types of handguns as well as the marks made on bullets fired from them. Waite later invented the comparison microscope, which forensic scientists use to make side-by-side comparisons of the marks on two bullets at a time.

In the twenty-first century, forensic ballistics examinations are undertaken in virtually every criminal case involving firearms in the United States. The two basic types of weapons involved in forensic ballistics cases are handheld weapons (handguns or pistols) and shoulder weapons (rifles). The two types of firearms produce unique marks on bullets and shell casings when fired. Even after a weapon has fired hundreds of rounds, a bullet from that weapon will still match the first bullet from its barrel. For experts in forensic ballistics, bullet marks are like fingerprints; each firearm leaves marks that are unique to that weapon.

Forensic Techniques

Experts in forensic ballistics perform many different kinds of analyses, including making bullet comparisons, matching projectiles to weapons, and estimating the lengths of projectile flights, which enables them to determine the types of weapons used and the locations of the operators of weapons when they were fired. During investigations of crime scenes involving shootings, ballistics experts analyze the impacts of bullets on victims, whether wounded or dead, to determine the types and sizes of projectiles fired and the types of weapons used, the distances from the shooters to the victims, and the angles at which the shots were fired.

If bullets, cartridges, or cartridge cases are not found at the scene of a fatal shooting, a forensic pathologist will usually analyze the victim’s wounds to determine information about the type of weapon used. Entry wounds are generally smaller than exit wounds and have dark rings around the injured surfaces, and by examining these, experts can often determine the width and thus the likely caliber of the bullets that made the wounds. This technique is referred to as wound ballistics.

When bullets are recovered from crime scenes, ballistics experts compare the striations on the bullets to those on other bullets from known sources. If the firearm suspected to have been used in a given crime is available, a test bullet is shot from that weapon and then the marks on that bullet are compared with the marks on the bullets found at the crime scene. The bullets found at crime scenes are also often compared with thousands of images of bullets stored in law-enforcement databases. Matches to bullets in such databases can give investigators important information about the histories of the weapons that fired the bullets.

The identification of specific weapons is another important aspect of the forensic investigation of crimes involving firearms. Many criminals remove the serial numbers from the guns they use—by filing the numbers off or using acid washes—because they believe this will make the weapons untraceable. Forensic scientists, however, are able to reclaim obliterated serial numbers using sophisticated techniques. To recover a gun’s missing serial number, the examiner files down the metal that carried the serial number to retrieve a strip of highly polished and hardened metal located beneath where the original serial number was stamped. By adding a solution of copper salts and hydrochloric acid to the area, the scientist can dissolve the weaker metal below where the numbers were stamped to reveal an imprint of the original serial number. This imprint is then photographed before the metal dissolves completely, and the photograph serves as documentation of the weapon’s serial number.

Related to the work of ballistics experts is the detection and evaluation of gunshot residue, which figures importantly in forensic investigations. The amount and scatter of provides information about the proximity of a victim to a weapon as it was fired. In addition, gunshot residue on the hands, skin, hair, and clothing of persons who were present at the time of a crime can reveal how close those individuals were to the weapon. Firearms give off a back-spray of gunpowder when discharged, and this hot and sticky substance adheres to most items of clothing and skin with which it comes in contact. It may remain embedded in objects during subsequent and sometimes repeated washings or cleanings. Forensic scientists sometimes use electron scanning techniques to detect minute particles of gunshot residue on watches and other jewelry worn by people suspected of having used guns in crimes.

New technology in the 2020s made forensic ballistics more accurate than ever before. Forensic ballesticians began using high-resolution microscopy, computerized databases of firearms and ammunition, and 3D-imaging techniques. Using digital imaging tools, forensic specialists were able to identify ballistic evidence faster and with greater accuracy.

Bibliography

Carlucci, Donald E., and Sidney S. Jacobson. Ballistics: Theory and Design of Guns and Ammunition. CRC Press, 2008.

Heard, Brian J. Handbook of Firearms and Ballistics: Examining and Interpreting Forensic Evidence. John Wiley & Sons, 1997.

Hueske, Edward E. Practical Analysis and Reconstruction of Shooting Incidents. 2nd ed., CRC Press, 2016.

Khan, Imtiyaz. "Unveiling the Mysteries of Forensic Ballistics: Solving Crimes One Bullet at a Time." Journal of Forensic Medicine, vol. 8. no. 2, 2023, www.hilarispublisher.com/open-access/unveiling-the-mysteries-of-forensic-ballistics-solving-crimes-one-bullet-at-a-time-99959.html. Accessed 13 Aug. 2024.

Zukas, Jonas A., and William P. Walters, eds. Explosive Effects and Applications. Springer, 1998.