DNA analysis of fired cartridge casings has been studied since the early 2000s and continues to be an emerging field in forensic investigations. While cartridge casings have been traditionally examined for fingerprints and tool marks, recent advancements in collection, packaging, and processing methods have opened new possibilities for recovering usable DNA profiles from these common items of evidence.

Early attempts to recover DNA from cartridge casings met with limited success. In a November 2022 Forensic Science International paper, the authors speculated that forensic DNA analysis of cartridges and fired cartridge casings was challenging due to “the heat and pressure generated during the firing of a weapon as well as metal ions from the casings that have been suggested to initiate DNA degradation and inhibit polymerase chain reaction (PCR) during the DNA profiling process.”

However, recent studies and technological advancements have significantly improved the potential for obtaining usable DNA profiles from these challenging samples and are removing barriers to the analysis of cartridge casings recovered from crime scenes. Key improvements include enhanced extraction and amplification methods that allow for the detection and analysis of smaller quantities of DNA. Modified protocols to handle low-template DNA samples are another improvement that some labs have implemented. In a third improvement, probabilistic genotyping software allows the analysis of challenging samples that contain complex, low-level and mixed DNA profiles. Lastly, new collection devices and extraction protocols allow maximum DNA recovery from metal surfaces of cartridge casings, including techniques like soaking and sonication to dislodge cellular material.

The importance of proper handling and packaging of evidence is crucial. Improper handling, packaging, and transport of casings significantly impacts DNA recovery. Placing casings in paper envelopes or cardboard boxes can lead to DNA transfer and loss. To address this issue, manufacturers have developed specialized tools and packaging solutions, such as the Case Grabber, which ensures safe collection, packaging, and transport of casings.

The author of the paper “Collecting, Transporting and Processing DNA on Fired Cartridge Casings”cites the following best practices for crime scene investigators to maximize the chances of recovering usable DNA from fired cartridge casings:  

Prompt Collection: Casings should be collected as soon as possible to minimize DNA degradation.

Proper Handling and Packaging: Use appropriate personal protective equipment and avoid touching the surfaces of the casings. Consider picking up the casing from the inside and using packaging materials that prevent contact with the casing’s outer surface.

Effective Swabbing: A double swab technique, using one wet followed by one dry sterile swab, can effectively sample the entire surface of the casing.

Optimized Laboratory Techniques: Labs should use DNA extraction methods optimized for low-template samples and highly sensitive PCR amplification kits designed for touch DNA samples.

Probabilistic Genotyping Software: This software can help maximize the evidentiary value of complex, low-level DNA profiles.

The future of DNA analysis on cartridge casings is encouraging. As technology continues to advance, the potential for recovering DNA from cartridge casings is expected to improve further. Promising areas of research include techniques that allow for direct DNA amplification from the evidence item, bypassing the extraction step. 

The ability to link a suspect directly to a crime scene through DNA on a fired cartridge casing can be a game-changer in forensic investigations. While not every casing will yield a usable profile, the potential to uncover critical evidence justifies the effort of routine DNA swabbing of these small but potentially significant pieces of evidence.   

Sources: Forensic Magazine, Forensic Science International November 2022

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