Firearms and Toolmark Identification

The field of forensic sciences has experienced a recent surge of public interest, most of the principles the field was founded upon are not new. This holds especially true for forensic firearms identification. Although firearms identification has made remarkable technological advances, many of the same techniques and instruments which were first implemented over 50 years ago are still in use today.

Single ImageThe primary tool of every forensic firearms examiner, the comparison microscope, has changed little since it was adopted in 1925. This instrument is justifiably considered the most significant contribution to the field, since without it firearms identification would be impossible. The comparison microscope is actually composed of two individual microscopes connected by an optical bridge. This allows the examiner to view two objects simultaneously with one-half of the field of view magnified by one microscope while the other half is magnified by the second microscope. As a result of this development, identification of fired bullets and cartridge cases became as conclusive as fingerprints.

Identification of fired ammunition to a specific firearm is made possible by distinguishing marks left by Single Imagethe gun on the surface of both the bullet and the cartridge case. These markings fall into two categories: class characteristics and individual characteristics. Class characteristics include general features such as the number of lands and grooves and their width, direction of twist, and bullet diameter. Since many firearms are built to the same specifications, the variation of these characteristics will be nominal from multiple guns of the same make and model. However, the individual characteristics imparted to a bullet or cartridge case are different for every firearm, and make it possible for an examiner to positively identify Single Imagea round of fired ammunition as coming from a specific gun.

Individual characteristics can be further classified into impressed or striated marks. Impressed marks are made by a hard surface imprinting its shape into a softer material. These marks include those made by a hammer striking another piece of metal, or a screwdriver driven straight down onto another surface. Striated marks are cause when a hard surface scrapes against a softer surface, such as pry marks made by a crowbar or when the surface of the bullet is forced along the rifling inside a gun barrel. These marks can be present on cartridge cases, bullets, and other tools. Comparison of the marks on the evidence with those on the test material can result in a "match", in which the striations on the two objects are synchronized and appear to form a single image.

Firearm and Toolmark Services

  • Firearms examination
  • Safety exams
  • Function tests (trigger pulls, etc.)
  • Test firing
  • Comparisons to test bullets and evidence bullet
  • Bullet Examination: If the only evidence available is a bullet, the bullet can be examined and a possible gun list can be determined from the general rifling characteristics on the bullet.
  • Caliber determination
  • GRC analysis (general rifling characteristics)
  • Collection of red/brown stains (blood) from items of evidence
  • Gunpowder residue analysis
  • Muzzle to target distance studies
  • Toolmark examination
  • Fracture match comparisons
  • Shooting scene reconstruction
  • Court testimony and presentations

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