Comprehensive Study Guide on Firearms Analysis and Forensic Ballistics

Responsibilities and Functions of the Firearms Analysis Unit

The Firearms Analysis Unit, under the direction of Garloff for ANTH 357, is tasked with a comprehensive suite of forensic duties essential to criminal investigations. The unit performs a meticulous inventory of all firearms and related evidence submitted to the laboratory. Beyond simple inventory, they are responsible for the long-term maintenance and storage of this evidence, ensuring that the chain of custody (COC) remains intact and legally defensible. A core technical function of the unit involves conducting operability reports to determine if a weapon is functional and safe to fire. Specialized forensic techniques are employed for serial number restorations on weapons where identifiers have been obliterated. Furthermore, the unit is responsible for National Integrated Ballistic Information Network (NIBIN) entries and the microscopic examination of cartridge cases and projectiles to establish forensic links between evidence and specific weapons.

Fundamental Rules of Firearms Safety

Safety is the paramount consideration when handling any firearm, governed by three major rules that must be strictly followed. First, a firearm must always be kept pointed in a safe direction at all times to prevent accidental injury or death. Second, the handler must keep their finger off the trigger until they are absolutely ready to shoot. Third, a foundational principle of firearm handling is to always treat a firearm as if it is loaded, regardless of its perceived status. These protocols are designed to mitigate the inherent risks associated with ballistic evidence and weaponry.

Defining Firearms and Operational Mechanics

A firearm is defined as an assembly consisting of a barrel and an action through which one or more projectiles are propelled by means of combustion. Within the forensic context of homicides, handguns are identified as the most common type of firearm utilized. The "action" of a firearm refers specifically to the mechanical method by which the weapon cycles and is operated. Long arms generally include rifles and shotguns, while handguns are subdivided into categories such as revolvers and pistols.

Characteristics and Evidence Collection for Revolvers

A revolver is a specific type of handgun characterized by several chambers that rotate around a central axis. In this design, discharge occurs successively using the same firing mechanism as each chamber aligns with the barrel. Common variations of revolvers include the Swing Out, Pin Type, and Top Break models. A major takeaway for forensic investigators is that after a revolver is fired, the spent cartridge cases remain within the cylinder and do not automatically eject; they must be removed from the firearm manually. When collecting revolver evidence, investigators must photograph the weapon in place before handling and use extreme care during pickup. It is strictly forbidden to place any object into the barrel of the weapon. Before packaging, the firearm must be rendered safe. Specifically for revolvers, the chambers must be photographed as found before removing any rounds. The chamber aligned with the barrel is marked as chamber number $1$, and each round is subsequently removed and documented.

Characteristics of Pistols, Rifles, and Shotguns

Pistols are handguns where the chamber is integral with the barrel, distinguishing them from revolvers. Cartridges for a pistol are loaded into a magazine, which is then inserted into the firearm. Unlike revolvers, the major takeaway for pistols is that after firing, the cartridge cases are automatically extracted and ejected from the firearm. Rifles are defined as firearms featuring a rifled bore, designed to be fired from the shoulder. Common rifle actions include bolt action, lever action, and semiautomatic. Shotguns are long guns also designed to be fired from the shoulder, but they typically possess a smooth bore and are designed to fire shotshells rather than single bullets. Shotguns chamber shotshells and, because they lack rifling, they are described as smooth-bored. Common shotgun actions include pump action and break open designs, the latter of which may be single shot or double barreled.

Anatomy of Firearm Bores and Components

The internal anatomy of a firearm barrel, or the bore, differs significantly between weapon types. Rifles and handguns feature a grooved bore. These spiral cuts, known as grooves, create ridges of metal between them known as lands. This rifling imparts a spiral spin on the bullet, which ensures greater aerodynamic stability and accuracy during flight. Conversely, the bore of a shotgun is smooth; rifling is omitted because it would cause the shot pattern to spread too early, reducing effectiveness. The general anatomy of a firearm includes the front sight, rear sight, muzzle, barrel, trigger guard, trigger, magazine, grip, and the firing mechanism components such as the hammer, bolt face, ejector, extractor, and firing pin. The projectile itself is the bullet, which is seated within the cartridge.

Composition and Categories of Firearm Ammunition

Firearm ammunition consists of four primary components. The cartridge case (or shotshell for shotguns) serves as the container for all other components. The primer is the ignition component, consisting of a high explosive sensitive to heat and shock. The propellant, or gunpowder, generates the necessary gas when ignited by the primer to move the projectile. The projectile is the actual object propelled by the expanding gases. A cartridge is a unit of ammunition for revolvers, pistols, and rifles consisting of the case, primer, propellant, and one or more projectiles. A shotshell is the unit of ammunition for shotguns and may contain a single projectile, known as a slug, or multiple pellets, known as shot. Jacketed ammunition features a metal coating, typically copper or brass, surrounding a lead core. This jacketing reduces barrel fouling, increases accuracy, and enhances bullet integrity upon impact. Common examples include Full Metal Jacket (FMJ), Soft Point Bullets (SP or JSP), and Jacketed Hollow Points (JHP).

The Mechanical Process of Firearm Discharge

The functioning of a firearm begins when the trigger is pulled, released the firing pin to strike the head stamp of the cartridge. This strike impacts the primer, which contains percussion-sensitive explosives such as Barium, Antimony, and Lead. This impact produces hot gases and sparks that ignite the gunpowder. As the gunpowder burns, pressure builds rapidly within the cartridge case, providing the force required to dislodge the projectile from the casing. Finally, the projectile travels down the barrel and exits the muzzle. This process is documented in various educational resources, including those provided by Merriam-Webster and specialized ballistic videos.

Forensic Classification and Individualization of Projectiles and Casings

Forensic examiners distinguish between class and individual characteristics. For projectiles, class characteristics include the caliber, color/finish, presence and type of jacketing, the manufacturer, the number of land and groove impressions, and the direction of the twist of those impressions. Individual characteristics on projectiles are found specifically in the land and groove impressions, which are unique to a specific barrel. Notably, because shotguns are smooth-bore, their projectiles rarely exhibit individual characteristics. For cartridge casings, class characteristics include construction/composition, finish, the location of the primer (center fire vs. rim fire), manufacturer, length, and the shape of the body and neck. Individual characteristics on casings include firing pin impressions, breechface marks, extractor and ejector marks, and chamber marks. These marks are mechanically impressed or scratched onto the casing during the firing and cycling process.

Firearms Operability Testing and Serial Number Restoration

Operability testing involves a rigorous rundown to ensure the firearm matches its identification. This includes identifying all evidence received, checking safety features, and inspecting the frame condition. A test fire is conducted to determine if the gun functions (goes "bang") using the proper caliber ammunition. If a firearm fails to fire, the examiner must determine the cause. Results are compiled into a report, administratively reviewed, and released. Cartridge cases from semiautomatics are typically collected for NIBIN entry. Serial numbers, required on frames or receivers by the $1968$ Gun Control Act, are manufacturer-applied markings for ID purposes. If these numbers are filed off by criminals, they can be restored using ultrasound, chemical etching, or electrochemical etching. Ultrasound uses cavitation in a high-frequency bath to reveal the numbers. Etching involves applying chemicals that react more quickly with the disrupted metal where the numbers were stamped, allowing them to be visualized. For test firing without damaging the projectile, agencies like the BPD (Baltimore Police Department) use water tanks to collect spent rounds.

Ballistic Identification Systems and Investigative Outcomes

Forensic ballistics utilizes two primary digital systems: the Integrated Bullet Identification System (IBIS) and the National Integrated Ballistic Information Network (NIBIN). IBIS is used to digitally image fired projectiles and casings, while NIBIN is a national database managed by the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) that stores these images to provide leads across jurisdictions. A "Lead" is generated through a visual comparison in the IBIS Matchpoint system, suggesting items could potentially be from the same firearm. A "Hit" is a confirmed match that occurs only after a microscopic examination of the physical evidence by an examiner, usually following a detective's request. This analysis allows investigators to link a cartridge case from a scene to a specific gun, link multiple scenes to each other via shared cartridge cases, or link a scene casing to a firearm found in a completely different location. The study of these mechanics and identifications is critical for understanding firearm injuries and solving ballistic-related crimes.