Firearms Unit

Forensic Scientists assigned to the Firearms unit conduct examinations of firearms and tool mark evidence to determine if a firearm or tool was used in a crime. Suspected firearms or tools are compared to evidence cartridge cases, bullets, or tool marks using a comparison microscope.

Other examinations performed include firearms function testing, distance determinations, ejection pattern testing, obliterated serial number restorations, and shooting incident reconstructions.

In addition, examiners enter fired cartridge cases into the National Integrated Ballistic Information Network (NIBIN), which contains images of fired cartridge cases from crime scenes, as well as test fires from guns used in crimes. When new images are entered into the system, images are searched in the database for a match. Members of this unit also testify in court.

Terminology

Terms commonly used by firearms examiners:

decorative
Pistol
A handgun in which the chamber is part of the barrel.

decorative
Revolver
A firearm having a breech loading chambered cylinder.

decorative
Cartridge
An unfired unit of ammunition consisting of the bullet, cartridge case, primer, and gun powder.

decorative
Bullet
A non-spherical projectile for use in a rifled barrel.

decorative
Projectile/Projectile Fragment
Any object projected by external force (e.g.; bullet, slug, BB, etc.)

decorative
Cartridge Case/Casing
The container for all the other components which make up a cartridge. May be fired or unfired.

decorative
Magazine
A container for cartridges which ahs a spring and follower to fee those cartridges into the chamber of a firearm.

decorative
Clip
A device used to load/charge a magazine. A magazine is not a clip.

Examinations Performed

Trigger Pull Testing - How much force is required to pull the trigger and discharge this firearm?

Function Testing - Does this firearm function as designed?

Full-Auto Testing - Is this firearm capable of full automatic fire (multiple shots with just one pull of the trigger)?

Accidental Discharge Testing - Will this firearm unintentionally discharge?

Shot Pattern/Distance Determination Examinations - What was the distance between the muzzle and the target at the time of discharge? This exam is conducted with the same firearm and type of ammunition used in the original shooting.

Gunshot Residue Test Kits - Are there microscopic GSR particles present on a suspect’s hands?  This test can help to determine whether a person was involved in a shooting.

Ejection Pattern Testing - For a specific firearm, where do fired cartridge casings land? The answer can help place and/or position the shooter at a crime scene.

Trajectory Reconstructions -What was the path of this projectile? Where did this shot originate?

Serial Number Restorations - What was this firearm’s serial number before it was altered/obliterated?

Comparative Examinations - Were the tool marks left on an evidence item (bullet, cartridge case, tool mark) produced by a particular suspect firearm or tool? Were the tool marks left on evidence items produced by the same unknown firearm or tool

NIBIN Program

The National Integrated Ballistic Information Network (NIBIN) is a national program administered by the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) that allows law enforcement agencies to share information, link cases, and obtain investigative leads in firearms related crimes. When a cartridge case is fired, the gun leaves behind markings on the cartridge case. These markings can be used to identify the cartridge case back to the gun that fired it. The NIBIN system makes use of these markings by taking a digital image of the evidence and searching it against the database of crime scene cartridge case images for similar markings. The system returns a list of possible candidates for comparison and a Firearms Examiner reviews each correlation to determine if there are any hits. If a hit is found the examiner confirms the hit by comparing the evidence on a comparison microscope. Mesa PD entered a partnership with the ATF in 2003 and an image acquisition station or Integrated Ballistic Identification System (IBIS) was deployed to the Mesa PD Crime Laboratory.

NIBIN Advantages:

  • Allows Mesa PD to link shootings to each other and to seized firearms.
  • Crosses jurisdictional boundaries and allows multi-agency searches.
  • Entered images stay in system and are searched against new entries.
Back to top