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What is cartridge cases – in forensic science
admin to Forensic Science  



Also called shells or casings, the cartridge is the part of a round of ammunition that encloses the propellant. The bullet is seated at the forward end and the primer at the base of the cartridge case. Casings are usually made of brass (which can be reloaded), nickel-coated brass, or aluminum, which is not designed to be reloaded. Given the composition, cartridge casings are often referred to generical-ly as brass. In shotgun ammunition, the cartridge case is made of plastic or cardboard and crimp-sealed at the top.

Cartridge casings used in revolvers are manually removed while cartridges fired in semiautomatic and automatic guns are ejected by the gases created when fired. Rifle cartridges are generally distinguished by their longer size and tapering in the neck. Cartridges may have cannelures around their circumference to act as a seat for the bullet. Cartridges are also differentiated based on what type of primer is used; rimmed primers are used in smaller caliber weapons such as.

22 while center-fire cartridges are used in larger caliber weapons. The base of a cartridge is often labeled in some way, such as the manufacturer and/or caliber of weapon that the ammunition is intended for.

When cartridge casings are recovered at a crime scene, where they are found can provide critical information. In the case of automatic and semiautomatic weapons, the direction of ejection of the cartridges, elevation (relatively flat compared to arcing), and side of the weapon discharged from can provide useful information. Depending on the circumstances and other evidence recovered, position of the recovered cartridges can help determine this information and thus point to possible types and makes of weapons. When cartridge cases are examined in the firearms section of the forensic lab, usually the first types of evidence sought are latent fingerprints and trace evidence such as dust, dirt, hairs and fibers, blood, and so on, that might be adhering to them. Once this examination is complete, the firearms expert will conduct a microscopic analysis using a comparison microscope. If a suspect weapon is available, comparisons can be made using cartridges recovered from test firing.

A typical examination begins with the determination of class characteristics such as stamps on the end of the cartridge and other markings that can indicate a manufacturer, a potential source of the ammunition, and a type of weapon in which the cartridge was used. There are cases where weapons are modified to fire cartridges not designed for them, which can complicate this initial investigation. Another factor that the examiner must consider is the possibility that the cartridge has been reloaded and used more than once in one or more different weapons. Each firing will add additional and distinctive marks atop those already there.

If a suspect weapon is available, the next step is to obtain comparison cartridges from that same weapon using ammunition as nearly identical to that originally submitted as evidence as possible. The goal of this comparison is to individualize the cartridge and to determine conclusively if the cartridge was or was not fired from the recovered weapon. Every weapon will leave marks of some type on a cartridge casing, and the list of possibilities includes breech face markings, firing pin impressions, extractor/ejector marks, chambering marks, marks made by or in the magazine, and marks due to the expansion of the casing against the walls of the chamber. Ejector, extractor, and magazine marks are created only in automatic and semiautomatic firearms, and not all cartridges will bear all these types of marks. Under the comparison microscope, the firearms examiner will attempt to determine if the markings from the test cartridges can be matched up to those on the cartridge casings submitted as evidence.