Forensic Ballistics
The prevalence of hand-gun related crime in the United States compared to most other developed countries provided the impetus for the development of the comparison microscope. As with most firearms, the fired ammunition components may acquire sufficient unique and reproducible microscopic marks to be identifiable as having been fired by a single firearm. Making these comparisons is correctly referred to as firearms identification, or sometimes called as "ballistics".
Historically, and currently, this forensic discipline ultimately requires a microscopic side-by-side comparison of fired bullets or cartridge cases, one pair at a time, by a forensic examiner to confirm or eliminate the two items as having been fired by a single firearm. For this purpose, the traditional tool of the firearms examiner has been what is often called the ballistics comparison microscope.
The interior of a gun's barrel is machined to have grooves (called rifling) that force the bullet to rotate as it travels along it. These grooves and their counterpart, called "lands" imprint groove and land impressions on the surface of the bullet. Together with these land and groove impressions, imperfections on the barrel surface are incidentally transferred to the bullet's surface. Because these imperfections are randomly generated, during manufacture or due to use, they are unique to each barrel. These patterns or imperfections, therefore, amount to a "signature" that each barrel imprints on each of the bullets fired through it. It is this "signature" on the bullets imparted due to the unique imperfections on the barrel that enable the validation and identification of bullets as having originated from a particular gun. Comparison microscope is used to analyze the matching of the microscopic impressions found on the surface of bullets and casings.
When a firearm or a bullet or cartridge case are recovered from a crime scene, forensic examiners compare the ballistic fingerprint of the recovered bullet or cartridge case with the ballistic fingerprint of a second bullet or cartridge case test-fired from the recovered firearm. If the ballistic fingerprint on the test-fired bullet or cartridge case matches the ballistic fingerprint on the recovered bullet or cartridge case, investigators know that the recovered bullet or cartridge case was also fired from the recovered gun. A confirmed link between a specific firearm and a bullet or cartridge case recovered from a crime scene constitutes a valuable lead, because investigators may be able to connect the firearm to a person, who may then become either a suspect or a source of information helpful to the investigation.
Read more about this topic: Comparison Microscope