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 that individual weapons were used in the crimes.

A ballistic body is any object in motion (such as a bullet) that moves under the influence of forces like gravity and air resistance after being launched. 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 trajectory influence the characteristics of entry and exit wounds, though wound appearance can vary widely depending on multiple factors.

The science of firearms ballistics is divided into three main components—internal, external, and terminal ballistics—with sources additionally recognizing transition (or intermediate) ballistics as a separate phase. 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 ignition of the propellant, its discharge from the chamber, and its pathway through the barrel. Transition, or intermediate, ballistics is the study of the immediate effects on projectiles as they leave the barrel; this area of ballistics focuses on forces such as muzzle blast, air pressure, and initial yaw, 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, when a bullet’s markings (referred to as a bump on the bullet) were matched to a suspect’s bullet mold in a London murder case. It was found that the recovered bullet had a distinctive casting mark that matched a bullet mold found in the suspect’s home. When confronted with this evidence during questioning, the suspect confessed to the crime. Nearly seventy years later, in 1902, attorney Oliver Wendell Holmes, Jr., later a U.S. Supreme Court Justice, is said to have used a magnifying glass to compare striations on a test bullet he fired into cotton wool against those on a bullet recovered from a victim’s autopsy. 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. This marked an early, rudimentary form of ballistic comparison.

Shortly thereafter, ballistics experts, including 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, and later Goddard helped establish a formal bureau for firearm identification. Waite helped catalog firearm characteristics, and later the comparison microscope—developed by Philip O. Gravelle and popularized by Calvin Goddard—which forensic scientists used to make side-by-side comparisons of the marks on two bullets at a time.

In the twenty-first century, forensic ballistics examinations are widely used in many criminal cases 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 could still match the first bullet from its barrel, after factorizing for influences such as damage, wear, and corrosion. For experts in forensic ballistics, bullet marks are like fingerprints; each firearm is thought to leave distinct 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 reconstructing projectile trajectories, which enables them to determine the types of weapons used and the approximate 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 may analyze the victim’s wounds to determine information about the type of weapon used. Entry wounds are often smaller than exit wounds and may show an abrasion collar or reddish-brown rings around the wound edges; 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 hundreds of thousands of ballistic signatures 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 prepares and polishes the area where the number was removed and then uses chemical or physical methods to reveal deformation patterns left beneath the surface. By applying a suitable chemical etchant to the prepared area, forensic analysts 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 gunshot residue can provide 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. When a firearm is discharged, microscopic particles of burned and unburned residue are expelled, and they adhere to most items of clothing and skin with which it comes in contact. Some residue may remain embedded in objects after handling or partial cleaning, though washing often removes them. Forensic scientists sometimes use scanning electron microscopy with energy-dispersive X-ray spectrometry techniques to detect minute particles of gunshot residue on watches and other jewelry worn by people suspected of having used guns in crimes.

A 2025 study by researchers at the University of Amsterdam reported a new gunshot residue detection technique that makes lead particles visible at crime scenes, enabling faster, more effective on-scene forensic investigations. New technologies in the 2020s made forensic ballistics more accurate than ever before. Forensic ballisticians began using high-resolution microscopy, computerized databases of ballistic evidence, and three-dimensional (3D)-imaging techniques. Using digital imaging tools, forensic specialists were able to identify ballistic evidence faster and with greater accuracy. The use of machine learning and artificial intelligence in forensic ballistics is enhancing the accuracy and objectivity of firearm, bullet, and wound analysis by supporting examiner interpretation with computational tools.

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