The Firearms and Toolmark discipline is a versatile, well-equipped unit offering a number of services that can be useful to investigators.

Firearms and Toolmark Examiners are dedicated to providing reliable scientific support to all law enforcement personnel. Services are provided at both the investigation and trial-preparation stages of criminal cases involving the use of a firearm or other tool.

• The type of weapon that a particular bullet or cartridge case was fired from
• Whether a bullet was, or was not fired from a suspected weapon
• Whether a cartridge case was, or was not fired in a suspected weapon
• Whether a tool found in a suspect's possession was, or was not used to cut, scrape, pry, or pinch evidence material seized from a crime scene
• The original serial number of a weapon or other metal object after the number has been obliterated
• If gunpowder is present on a victim’s clothing or on other evidence that may have been the target of the suspect
• The distance from the muzzle of the firearm to the target at the time the weapon was fired
• If fingernail clippings or fragments found at a scene can be identified as having come from a specific person

Many other miscellaneous examinations may be performed at the request of the customer. Examiners in the Firearms and Toolmarks discipline may conduct other testing that is of special interest to an investigator. Such requests may be made at the time of evidence submissions or by phone.

Basics of Firearms Comparisons

Inside the barrels of handguns and rifles are spiral impressions called rifling. The raised portions of the rifling are known as lands and the recessed portions are known as grooves. When a weapon is fired, these lands and grooves cut into the bullet, putting spin on it as it travels through the barrel of a firearm. Because bullets have an oblong shape, spin is necessary for accurate flight.

The impressions of lands and grooves remain on the bullet after it has been fired.

Since rifling characteristics can differ from one firearm manufacturer to another, forensic firearm examiners can determine the type of weapon that fired a particular bullet by examining the impressions of the lands and grooves on the bullet. They examine the width, the number, and the direction of the twist of the lands and grooves. For example, a 9mm pistol made by one company might have a barrel with 6 lands and grooves that twist to the right and another company's 9mm might have 6 that twist to the left. In addition, the width of the lands and grooves may differ.

Because each barrel will have imperfections left by the manufacturing process that will leave unique marks on a bullet, firearm examiners can determine whether a bullet recovered from a crime scene or victim was fired from a weapon taken from a suspect.

Comparison Process

The first step in the Comparison Process is to test fire the firearm into a water tank in the lab.

< read the complete article and view example photographs >

• Firearm Identifivcation in the Forensic Laboratory, Robert M. Thompson
• The Science Behind Firearm and Tool Mark Examination, Nancy Ritter
• A Practical Guide to Shooting Scene Preservation for Crime Scene Investigators