77
Using Physics: Rifling and Ballistics
The jury looked over the evidence for hours, comparing the character-
istics on “Bullet III” and on the barrel of the Colt taken from Sacco. In the
end, the jury determined that the bullet had to have come from Sacco’s
gun.
When Guns Are
Used for Crime
When a crime involves the use of a firearm, it is important for the forensic
scientist examining the evidence to gather as much information as possible
about the gun used in the commission of the crime. Similar to the Principle
of Exchange, most guns
impart
a sort of fingerprint on every bullet that
passes through their barrel and on the spent shell casings of the fired car-
tridges. The term used for these identifying marks is rifling. This is the sort
of evidence that contributed to the Sacco and Vanzetti case.
It is often possible to use physical examinations of the gun, bullets, and
casings found at the crime scene to determine whether a specific gun was
used in the crime. Linking a gun to a crime scene is a good way to start
the process of connecting a particular person to the shooting. If ownership,
and more important, possession, can be established, the gun can become
very strong evidence against a suspect.
The identifying characteristics of a gun are dependent on the lands—
raised edges—and grooves the manufacturer builds into the inside of the
barrel to help improve the gun’s accuracy. A bullet that spirals as it comes
out of the barrel is much more accurate than one that does not. Gun manu-
facturers use unique patterns of lands and grooves based on their research,
so it is not uncommon for guns from two different makers to have different
ballistic patterns right out of the box.
