Criminalists assigned to the firearms section of the laboratory examine firearms and ammunition components to assist in criminal cases. Depending on the nature of the crime, cartridge cases, bullets, and firearms are recovered from crime scenes and submitted to the laboratory. Firearms examination involves the comparison of microscopic characteristics between fired bullets and fired cartridge cases to each other and to test fires to determine if they have a common origin. Put another way, it is an attempt to answer the question “Was this bullet or cartridge case fired in the firearm in this case?” To answer that question, an overview of ammunition, firearms, and microscopic comparison is needed.
First, ammunition is comprised of four major components: A cartridge case, a primer, a bullet, and gunpowder. The cartridge case can be made from different types of metal and acts like a housing unit for the other components. On the back of the cartridge case is where the primer is seated. During firing, the primer is struck by the firing pin and starts a chain reaction. Energy is released from the primer into the inner portion of the cartridge case where the gunpowder is held. The gunpowder burns rapidly, causing expansion in every direction which forces the bullet out of the cartridge case and down the barrel of the firearm; meanwhile, the cartridge case expands in every other direction, sometimes picking up microscopic details from the firearm in the process. Once the cartridge has been fired, the bullet is separated from the cartridge case. It is very difficult to find markings that can link a bullet back to the cartridge case it was once integral with, but potentially each ammunition component can be separately linked back to a firearm.
Second, a firearm can be described as a complex collection of tools that interact on ammunition components. Common parts in the firearm that interact with cartridges are the firing pin, breech face, chamber, extractor, ejector, and barrel. Each of these parts is created using different machining methods according to the needs of the manufacturer. During the machining process, each gun part receives microscopic characteristics based on the machining method used. These characteristics are then imparted onto cartridge cases and bullets during firing by either impressing the microscopic characteristics into the ammunition or by “scraping” across the surface of the ammunition creating striations. It is these characteristics that a firearms examiner compares to determine if items of evidence were fired in a particular firearm.
Lastly, microscopic comparison is what ties bullets and cartridge cases to the firearm. Microscopic comparison is comprised of two parts: class and individual characteristic examination. Class characteristics are dimensions or shapes decided by a firearms manufacturer prior to any firearms being created. For example, if a firearm has a rectangular firing pin it is because the manufacturer designed it to be rectangular. All discernible class characteristics between two items must agree to move forward to examining individual characteristics. If the class characteristics do not agree, the two items were not fired in the same firearm.
Individual characteristics happen incidental to manufacture and occur randomly when the pieces of a firearm are being created. It is also possible that parts of the firearm become altered during repetitive use or misuse. Again, take the firing pin as an example: it can start out as a larger piece of metal that is then shaped by removing metal using cutting instruments. As the metal is removed it forms metal chips at random intervals. Due to the random nature of this process, each firing pin has small random defects that have a unique shape, size, and orientation to one another. If there is agreement of the individual characteristics, then an examiner can conclude the items had a common origin.
So, let’s tie it all together. Imagine a crime scene where several cartridge cases were recovered, and sometime later through investigation, a suspect is established who is in possession of a firearm. A firearms examiner would test fire the firearm at the laboratory and record the class characteristics of the firearm. In this example, let’s say the firearm is a 40 S&W caliber pistol with a circular firing pin. The cartridge cases from the scene are also 40 S&W caliber cartridge cases and each has a circular firing pin impression. The examiner would see if the class characteristics are similar and if so, begin looking at any discernible individual characteristics. They notice there are several small burrs on the firing pin that correspond in shape, size, and orientation to areas in the firing pin impressions of the cartridge cases from the scene. Taking these into account, and by looking at any other area that may possess individual characteristics, the firearms examiner concludes the cartridge cases from the scene were fired in the suspect’s firearm.
Each case has its own challenges, ranging from damaged or fragmented ammunition components to keeping track of a large volume of evidence. No matter what the specific details of a case are, the same foundational procedure is followed: compare class characteristics, and if they agree, compare individual characteristics. By using a systematic approach, the firearms examiner can present scientific conclusions to describe the connection between ammunition components and firearms, giving voice to the evidence that cannot speak for itself.