What Are Gunshot Residue Kits and How Are They Used in Criminal Investigations?

Understanding Gunshot Residue (GSR) Testing

When a firearm is discharged, the bullet is pushed through the barrel by a small explosion that creates a cloud of tiny, microscopic particles. These particles include the elements used in the creation of gunpowder, primarily lead, barium, and antimony. These particles then land on the surfaces around the fired weapon and leave behind a residue. This residue, known as gunshot residue (GSR), can settle on the hands, clothing, or other surfaces near the person who fired the gun. Forensic investigators use gunshot residue kits to collect and analyze these particles, helping to determine whether an individual recently fired a gun or was in close proximity to a discharged firearm.

How Do Gunshot Residue Kits Work?

A GSR kit typically includes:

• Adhesive stubs or swabs to collect residue from a suspect’s hands, clothing, or other surfaces.
• Chemical reagents to test for the specific elements found in gunshot residue (lead, barium, and antimony).
• Collection instructions to ensure proper technique and prevent contamination.
• Evidence bags and documentation forms to maintain a chain of custody for use in legal proceedings.

The standard collection process involves:

1. Swabbing or dabbing both hands, the wrists, and possibly the face and clothing of a person suspected of firing a gun with an adhesive stub or moistened swab.
2. Properly labeling and sealing the collected samples to avoid contamination.
3. Sending the samples to a forensic laboratory like the Mississippi State Crime Lab, where they undergo analysis using:
   • Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS) – Identifies and confirms the presence of GSR particles.
   • Atomic Absorption Spectroscopy (AAS) or Inductively Coupled Plasma Mass Spectrometry (ICP-MS) – Detects the elemental composition of the residue.

The Role of GSR Kits in Criminal Investigations

GSR analysis is commonly used in homicide, assault, and weapons violation cases to determine whether a suspect:

• Fired a gun – Presence of GSR on the hands suggests recent firearm discharge.
• Was near a discharged firearm – GSR may also settle on bystanders or objects close to the shooter.
• Handled a recently fired weapon – Residue transfer can occur if a person touches a gun that has just been used.

Limitations and Challenges of GSR Testing

While GSR testing can provide critical forensic evidence, it has several limitations:

1. Contamination Risks – GSR can transfer from one person to another or onto surfaces, making it possible for an innocent person to test positive for GSR.
2. Environmental Factors – Elements found in GSR (lead, barium, antimony) may also be present in brake dust, fireworks, or industrial environments, leading to potential false positives.
3. Residue Persistence – GSR particles can removed by washing hands, wiping surfaces, or even normal activity within a few hours.

The Legal Impact of GSR Evidence

In Mississippi and across the U.S., GSR evidence is often used to support other forensic and circumstantial evidence rather than as standalone proof of guilt. Prosecutors may introduce GSR findings to demonstrate firearm use, while defense attorneys may challenge the results based on contamination risks or the absence of conclusive proof that the suspect actually fired a weapon.