Ballistics in Forensics - Caliber vs Gauge

When CSI's approach a crime scene and find a bullet with a certain caliber or gauge, just exactly what are they talking about? In this article, caliber and gauge will be defined to educate the reader so that the next time you watch your favorite CSI TV show, you will know what they are talking about.

What is a caliber?

Caliber is a term used toward firearms such as handguns and rifles. The caliber of a firearm refers to the size of the internal diameter of its barrel. Calibers of handguns and rifles are measured in millimeters or inches. A .25 caliber handgun has a barrel with an internal diameter of 0.25 inches or ¼ of an inch.

What is a gauge?

Gauge is a term used toward shotguns. The gauge of a shotgun also reflects the size of the internal diameter of the barrel. However, this size is not a direct measurement. Rather, the gauge of a shotgun is determined by counting the number of lead balls matching the barrel's diameter that it requires to weigh one pound. Therefore, it takes 20 lead balls to fill the diameter of a 20-gauge shotgun barrel and to weigh one pound.

The caliber and bullet type are crucial in ascertaining what firearm was used in a crime. If the bullet is intact, the police can determine the caliber by simply measuring it. Severely damaged and deformed bullets are weighed for the purpose of excluding some calibers but not for determining the exact caliber. For instance, the weights of .22 caliber and .44 caliber bullets differ slightly. Many times, forensic scientists can find out whether the bullet contains a jacket. If so, what kind of jacket? Making this determination helps to identify the bullet type and ultimately the weapon type used in a crime.

The shotgun pellets, or lead balls, from a shotgun can be removed from the victim during surgery or autopsy and from floors, walls, and other places. The size of the shot does not necessarily tell you the gauge of the shotgun but rather provides information about the ammunition used. The different sizes of shot are numbered. The smaller the number, the larger is the size of pellets inside the shotgun shell. For instance, Number 7 shot is smaller than Number 4. Pellets differ in size from 0.05 inches for Number 12 shot to 0.33 inches for 00 shot, the biggest, which also goes by the name "double-O" buckshot. A forensic ballistics examiner measures the diameter of any shot found and ascertains what size shot was used.

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