Forensic evidence includes many different types of evidence that are collected during a criminal investigation and used in a court of law. Typically, forensic evidence is collected and analyzed through scientific method: fingerprint evidence, bite mark evidence, DNA evidence, hair and fiber analysis, blood tests, and so forth.

Ballistics is the forensic science dealing with firearms and firearm projectiles. Specifically, the field of ballistics is concerned with identifying specific marks a firearm makes on a bullet, the angle of trajectory which a bullet travels after being fired, and the damage a bullet causes when it strikes a surface.

Ballistic evidence is used to identify the type of weapon that was used in the commission of a crime and other details of the crime—for example, where the shooter was standing in relation to his or her target. It may also tell whether a weapon used in one crime has been used in the commission of another crime where identical ballistic evidence was discovered.

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How is Ballistic Evidence Used in Court?

Any bullets left at the scene of a crime are collected by investigators and analyzed by forensic ballistics experts. These analysts examine “rifling” in the spent bullet, which can help identify the firearm—or at least the type of firearm—used in the commission of the crime.

When a gun is manufactured, spiraling “lands and grooves” are created inside the barrel of the gun. These ridges and indentations are known as “rifling,” and as a bullet passes through the gun barrel, the rifling of the barrel leaves characteristic markings on the bullet. Ballistics experts often conduct comparative examinations of two bullets to determine if their rifling patterns match and if they came from the same gun.

This can be useful in prosecutions by identifying a suspect based on his or her possession of the firearm used to fire the bullets found at the crime scene.

Ballistic evidence—including gunshot residue, angle of trajectory, distance from the target, bullet entrance and exit marks, and damage—are often used to reconstruct the events that transpired in the commission of a crime. For example, a person who says a gun accidentally fired may be proven truthful or untruthful by forensic evidence that shows the trigger pressure of the weapon, the angle of the gun when fired, and the distance from which the gun was fired.

Is Forensic Evidence Reliable?

For many decades, ballistics has been held as an indisputable science in courtrooms across the United States. However, defense attorneys have been challenging the reliability of ballistics for nearly as long as judges have been accepting it as evidence in their courtrooms.

An article published in the American Bar Association’s Criminal Justice Magazine highlights important court cases that show an increased skepticism of the reliability of such evidence. The article cites two court cases (United States v. Hicks and United States v. Foster) which both ruled against challenges to forensic approaches primarily because courts had accepted such evidence “for many years” or “for decades.”

However, more recently, challenges  have made more headway. In United States v. Green, the court ruled that the forensic expert could testify that casings in bullets were similar, but could not testify that the bullets came from a specific gun “to the exclusion of every other firearm in the world.” This assertion that ballistic evidence could not specifically and exclusively identify the exact weapon which fired a bullet has been upheld and reasserted in numerous court cases since.

In 2009, the National Academy of Sciences issued a report acknowledging the limitations of forensic ballistics, saying that there is too much variability among weapons to determine how many points must match in order to achieve a given level of confidence in a result, and that more studies are needed to solidify and this type of evidence in order to achieve reliability.