Criminology
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A firearm is a weapon which projects either single or multiple projectiles high velocity through rapid, confined burning of a propellant. This process of rapid burning is technically known as deflagration. Most modern firearms (with the notable exception of smoothbore firearms) have what is called rifling barrels to impart spin to the projectile for improved flight stability. The rifling also helps in forensic cases because its like a guns finger print. Rifling consists of grooves cut or formed in a spiral nature, lengthwise down the barrel of a firearm.
Rifling is placed in the barrels of firearms to impart a spin on the bullets that pass through it. Because bullets are oblong objects, they must spin in their flight, like a thrown football, to be accurate. Looking down the barrel of a firearm you might see rifling like that depicted on the right. This image shows a pattern of rifling containing six grooves with a right twist.
In firearm examiner lingo we refer to the rifling as lands & grooves. The lands are the raised areas between two grooves. A rifling pattern of eight grooves with also have eight lands. Firearms can be manufactured with any number of lands and grooves in their barrels. They can also spiral either left or right. A few of the more common rifling patterns are 4/right, 5/right, 6/right, 6/left, 8/right, and 16/right. The procedures described below are abbreviated somewhat but I hope that they will provide you a better understanding of basic rifling techniques. When barrels are manufactured, they start out as a solid rod of steel. A hole is drilled down the center of the rod and the rifling is then placed in the barrel. There are three basic machining processes that modern firearms manufacturers use to form the rifling in barrels. Rifling can be cut into the inner surface of a barrel using a broach, the rifling can be formed using a hardened steel button, or the rifling will be formed through a process called hammer forging. A newer method of rifling barrels, called Electrochemical Rifling, does not involve the normal machining processes of the other techniques.