When you watch crime shows like Law & Order: Criminal Intent, you often hear detectives discussing the gun used to commit a crime. But how do they know what gun was used? The only evidence at the crime scene might be a bullet or a casing. How can this be enough information to identify a weapon?
It turns out that every gun produces different microscopic characteristics when it fires. The singularity of the characteristics is brought about by minor differences in manufacturing processes each weapon goes through, as well as how the weapon is used after it is made.

The uniqueness of the evidence that firearms leave behind makes it possible to use information from a crime scene to either connect a firearm to the scene or eliminate a firearm from consideration.

Firearm identification is the process of analyzing the bullets and cartridge cases left at a crime scene to determine if they came from a particular firearm. Marks on the bullets and cartridges may be common to every firearm of that type (for example, the caliber of the firearm). These are called class characteristics. Alternately, marks may be specific to that particular firearm (for example, corrosion of the rifling pattern contained in the barrel of the firearm.) These are called individual characteristics. A firearm is identified if there is sufficient agreement between the marks left at a crime scene and the marks made by the firearm in question.

Before firearms can be analyzed, firearm evidence must be collected. When a crime is committed, crime scene investigators must look in and on walls, floors, ceilings, trees, mattresses, etc. to find fired bullets and cartridge cases. In addition, they must look for evidence that might contain gunpowder or other residues, such as clothing with bullet holes.

After locating firearm evidence, crime scene investigators must document in detail its location and condition. Once the evidence is collected, crime scene investigators begin a meticulous record of the individuals who handle or transport the evidence as it makes its way to the forensic laboratory, where firearm identification occurs.
Forensic scientists who analyze firearm evidence in the lab are typically called firearm examiners.

The firearm examiner’ss job begins when firearm evidence arrives at the lab from the scene of the crime. The firearm identification process consists of several steps. First, the examiner may need to remove trace evidence from a bullet or cartridge for analysis in another part of the lab. Then, the examiner evaluates the class characteristics of the evidence.

When a firearm is submitted for comparison, the examiner first determines that its class characteristics match those of the evidence. If so, the examiner must discharge the weapon in a controlled environment to obtain casings and bullets from that weapon. The examiner then compares the individual characteristics of the laboratory-created cartridge cases and fired bullets to those of the evidence in question. Only then can the examiner determine if they are from the same source.

The firearm examiner must take detailed notes during every step of the process. These notes are used to write a full report about the analysis and its conclusions. If the firearm evidence is used in a case that goes to trial, the firearm examiner may be required to testify in court about the work. Firearm evidence is among the most important evidence in many cases!

Firearm and identification is a continually developing field of forensic science that has been around for more than 150 years. The opportunity for innovation in this field will undoubtedly continue for far longer than the next 150 years!

About the author: Emily Nelson earned an M.S. in Electrical Engineering from the Massachusetts Institute of Technology before beginning her career as a science writer.