In recent weeks we have looked at forensic evidence presented at trial, and how, though many jurors feel science is infallible, these types of evidence are sometimes far from solid. In some cases, the evidence presented as indisputable fact later becomes discredited as "junk science" when it becomes apparent just how flawed the theories behind the science can be.
Recently, we looked at fingerprint evidence to explore the limitations of using fingerprint analysis to determine a specific person's involvement in a crime. Today, we look at ballistic evidence.
Ballistics is a field of forensic science that pertains to firearms and their bullets. The study of ballistics seeks to identify the marks a weapon makes on a bullet as it fires, the angle of trajectory of the bullet, and the type and extent of damage the bullet makes when it strikes an object.
In criminal cases, investigators use ballistic evidence to determine things like the type of weapon that was used in a crime, the location of a shooter, and whether or not the weapon used in one crime bears similar markers to evidence found in another crime.
Bullets found at a crime scene are analyzed by forensic ballistics experts who examine the "rifling" of a spent bullet. "Rifling" is the term given to lands and grooves within the barrel of the gun in manufacturing. When a bullet is fired, the rifling leaves markings on the bullet as it passes through the barrel. These markings can be used to identify the type of weapon involved. A comparative examination of bullets from two crime scenes can help investigators determine if the same gun was used in multiple crimes.
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.
In addition to identifying the type of weapon--or in some cases, the specific weapon--used in a crime, ballistics can help determine how a crime was committed. In a murder case, for example, a witness might claim a shooting was in self defense as the shooter and the victim struggled for control of a gun. However, the angle of trajectory and the distance from which the gun was fired might show that the shooter was not at close range when the gun went off, but rather fired from a distance.
But just like fingerprint evidence, ballistics evidence leaves room for error, and its reliability is not without question.
Just because courts have accepted the evidence "for decades," does not mean that they should accept it without close examination of its uses and limits.
One court case, United States v. Green, shows that while ballistics evidence may show similarities of markings, these similarities--like with fingerprint evidence--cannot concretely identify one specific weapon "to the exclusion of every other firearm in the world."
And like fingerprint analysis, determining how many points must match to identify a specific weapon with any confidence is almost impossible. Even a national committee to assess the feasibility of a national database of ballistic imaging admits that ballistic evidence is not without challenges and limitations. In the executive summary of the study, the authors write, "The validity of the fundamental assumptions of uniqueness and reproducibility of firearms-related toolmarks has not yet fully been demonstrated."
If the experts aren't convinced, then neither should be a jury.
Image credit: Carl Wycoff