FRSC 1011H Chapter Notes - Chapter 9: Rifling, Firing Pin, Comparison Microscope
2 May 2018
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Firearms, Tool Marks, and Other Impressions
Types of Firearms, Bullet and Cartridge comparisons
● The manufacture of a gun barrel requires impressing its inner surface with spiral
grooves, a step known as “rifling”. Rifling imparts spin to the projectile when it is
fired, which keeps in on an accurate course.
● No two rifled barrels have identical striation markings. These striations form the
individual characteristics of the barrel. The inner surface of the barrel of a gun
transfers its striation marking to bullets that pass through it.
● The class characteristics of a rifled barrel include the number of lands and
grooves and the width and direction of twist.
● The comparison microscope is a firearm examiners most important tool because
it allows two bullets to be compared simultaneously.
● The firing pin, breech face mark, and ejector and extractor mechanism also offer
a highly distinctive signature for individualization of cartridge cases.
● Unlike handguns, a shotgun is not rifled- it has a smooth barrel. Because of this,
shotgun shells are not impressed with any characteristic rifling striation markings
that can be used to compare two shotgun shells to determine whether they were
fired from the same weapon.
Automated Firearm Search System
● The advent of computerized imaging technology has made possible the storage
of bullet and cartridge surface characteristics in a manner analogous to
automated fingerprint files.
● Two automated firearm search systems are DRUGFIRE, developed by the FBI,
and IBIS, developed by the ATF.
● NIBIN is the National Integrated Ballistics Information Network, a unified firearm
search system that incorporates both DRUGFIRE and IBIS technologies.
Gunpowder Residues
● The distribution of gunpowder particles and other discharge residues around a
bullet hole permits an assessment of the distance from which a handgun or rifle
was fired.
● The precise distance from which a handgun or rifle was fired is determined by
carefully comparing the powder residue pattern on the victim’s clothing to test
patterns made when the suspect weapon is fired at varying distances from a
target.
Document Summary
The manufacture of a gun barrel requires impressing its inner surface with spiral grooves, a step known as rifling . Rifling imparts spin to the projectile when it is fired, which keeps in on an accurate course. No two rifled barrels have identical striation markings. These striations form the individual characteristics of the barrel. The inner surface of the barrel of a gun transfers its striation marking to bullets that pass through it. The class characteristics of a rifled barrel include the number of lands and grooves and the width and direction of twist. The comparison microscope is a firearm examiners most important tool because it allows two bullets to be compared simultaneously. The firing pin, breech face mark, and ejector and extractor mechanism also offer a highly distinctive signature for individualization of cartridge cases. Unlike handguns, a shotgun is not rifled- it has a smooth barrel.
