Types of Marks on Fired Cartridge Cases
Fired cartridge cases are often the most commonly recovered evidence at shooting scenes. While the bullet may travel far and remain unrecovered, the empty cartridge case is usually ejected and left behind, making it a crucial piece of evidence.
Introduction
When a firearm is discharged, the ammunition experiences a series of rapid mechanical interactions, leaving behind microscopic marks on the cartridge case. These marks are not random. They are the result of the interactions between the cartridge and internal components of the firearm, such as the firing pin, breech face, chamber, extractor, and ejector. Below are the main types of marks found on spent cartridge cases:
1. FIRING PIN IMPRESSION
The firing pin impression is one of the most prominent marks found on a fired cartridge case. This mark is formed when the firing pin strikes the base of the cartridge case to ignite the primer and set off the gunpowder inside. The forceful impact creates a dent or crater on the primer surface. The exact shape, depth, and pattern of this impression depend on the design and condition of the firing pin.
2. BREECH MARKS
Breech marks, also called breech face marks, are impressions or patterns found at the base of the fired cartridge case. These marks are formed when the cartridge case is pushed backward against the breech face of the firearm during firing. These markings typically show up as parallel striations or granular textures in the soft primer metal around the firing pin mark.
Importantly, no two breech faces are exactly the same, even among firearms of the same brand and model. Because of the inherent manufacturing variations and imperfections in firearms, these markings can conclusively link a cartridge casing to a particular firearm. This uniqueness allows the breech marks to serve as vital individualizing features in ballistic analysis.
Types of Breech Marks
Forensic ballistics experts may categorise breech marks into two categories, based on their formation and appearance:
Impressed Breech Marks: These are indentations or impressions formed when the cartridge case firmly contacts the breech face under high pressure, creating a negative impression of the breech face on the fired cartridge.
Striated Breech Marks: These are scratch-like patterns created if there's a slight movement of the cartridge case against the breech face during firing or ejection.
3. EXTRACTOR MARKS
Extractor marks are produced when the extractor of the firearm (a small claw-like mechanism), grips the rim of a cartridge case to eject it after firing. The forceful contact often leaves behind scratches, dents, or gouges on the case. These marks can vary depending on the design of the firearm and the condition of the extractor.
Extractor marks are typically found on the rim or the side of the cartridge case. These marks help distinguish between manual extraction (like in bolt-action rifles) and automatic ejection (seen in semi-automatic firearms). Additionally, unique wear or defects on an extractor can link a spent cartridge to a specific weapon, assisting in ballistic identification.
4. EJECTOR MARKS
Ejector marks are formed when the ejector mechanism strikes the cartridge case to expel it from the ejection port of the firearm. After the extractor pulls the case from the chamber, the ejector strikes the cartridge case and forcefully expel it from the firearm. This sudden impact often leaves a dent, crescent-shaped gouge, or arc-like impression on the rim or base of the case. These marks vary in visibility depending on the design of the firearm, ejection force, and firing conditions.
Ejector marks can link spent casings to a specific firearm, as manufacturing variations and wear patterns can make them distinctive. The specific location of the marks and distinct features (scratches, dents, or crescent shapes) provide crucial matching points, especially when comparing multiple cases from a crime scene. This helps confirm whether they were fired from the same weapon.
5. CHAMBER MARKS
Chamber marks result from the expansion and movement of the cartridge case against the interior walls of the chamber during the firing process. As the gunpowder ignites and pressure builds, the cartridge case momentarily swells and presses against the chamber walls. This action can leave behind fine striations, drag marks, or abrasions on the outer surface of the case. The nature of these marks depends on the texture of the chamber walls. Smooth chambers leave minimal markings, while rough or dirty ones may leave more defined patterns. Although chamber marks are less distinctive than breech or firing pin impressions, they often serve as useful supporting evidence during microscopic comparisons.
Types of Chamber Marks
Banded Striae are horizontal or circular scratch lines around the body of the cartridge case.
Banded Spiral are similar to banded striae but in a spiral pattern.
Fluting Marks are straight, evenly spaced lines running along the length of the case. These marks are common in some firearms (like H&K rifles) that have fluted chambers to help with extraction.
Ramp Bulging is raised area on the case where it contacted the chamber ramp (the part that guides the cartridge into place).
Diagonal Marks are slanted lines or scratches that cross the cartridge case.
Rust or Corrosion Marks are irregular or rough spots on the cartridge case caused by rust, dirt, or debris inside a poorly maintained chamber.
6. OTHER MARKS
In some cases, additional markings may be present on the cartridge case. These can include magazine lip marks, which are created when the cartridges are pushed out of the magazine and into the chamber, and bolt face marks, which are found in bolt-action firearms where the case makes contact with the bolt during firing. While these marks may not be as consistent or prominent as the primary ones described above, they can provide further points of comparison, especially when the available evidence is limited.