Ballistics

A gun leaves an impression on the bullets that are shot from it. Forensic scientists can use these impressions to match bullets to the type of gun used. The field of science that studies bullets and other shot objects is called ballistics. Try the following activity to investigate how detectives can solve a crime by using their knowledge of ballistics to match bullets to a gun.

Materials
Ballistic evidence shown in the drawing

Procedure

1. Using the ballistic evidence shown, match the bullet found at the scene of a bank robbery with the sample bullets fired from several guns.
2. Which type of gun was used in the crime?

Explanation

The first thing that a forensic scientist will determine when examining a bullet is its caliber. A bullet’s caliber is the same as its diameter, the length of a straight line through the center of the flat end of the bullet. The diameter of a bullet is measured in either inches or millimeters. A bullet that is 0.22 inches in diameter is a .22-caliber bullet. It is used in .22-caliber rifle, which has a barrel 0.22 inches in diameter. Similarly, a 9-mm pistol uses a bullet that is 9 millimeters in diameter. Forensic scientists will compare the caliber of a bullet found at the scene of a crime to the caliber of a gun found on a suspect. But that is just the beginning of their ballistic analysis.

As early as the sixteenth century, gunsmiths were improving the speed and accuracy of guns by cutting grooves called rifling grooves around the insides of guns barrels. These grooves make the bullet spin down the barrel and in the air, thus increasing the speed and force of the bullet. Bullets are made slightly larger than the gun barrel to ensure a tight fit, which results in the characteristic rifling marks on the bullet after it has been fired.

Inside the gun barrel, the rifling grooves and the lands (the area inside the gun barrel that remains after the barrel has been grooved) create a distinctive pattern that does not change, so any bullet that passes through the barrel of a particular gun will bear the rifling marks of the gun. In addition to rifling marks made on the bullet, identifying marks are made on the cartridge (the cylindrical container of the bullet, which holds the gunpowder and stays in the gun when it is fired). These marks are caused by the firing pin (the piece of metal that strikes the cartridge and ignites the gunpowder).

Matching bullets to guns is one of the simplest procedures in forensic science. By the knowing the caliber of a bullet and the land and groove configuration, a forensic scientist can tell detectives what type of gun the bullet came from.

Detective Science in Action

Forensic evidence proves a person’s innocence as often as it proves a person’s guilt. Recently, for example, a prospector in Alaska was found in his remote cabin dead from a gunshot wound. The immediate suspect was the prospector’s partner. Indeed, when the police found the partner, he was carrying a recently fired rifle and had blood on his boots.

A hundred years ago, the partner would have been found guilty based on those two bits of evidence alone. But forensic evidence showed that the bullet that killed the prospector came from a pistol, not the partner’s rifle, and that the blood on the partner’s boots came from a recently killed deer.