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When a bullet is fired from a gun, the gun leaves unique markings, or grooves, on the surface of the bullet as it travels through the barrel. These grooves help forensic firearms examiners determine a match between the bullet and gun type and perhaps to the actual gun used in a crime.
What is a Rifling Pattern?
A spinning bullet is a more accurate bullet. Therefore, many guns have spiral grooves carved into the inside of their barrels to make the bullets spin as they leave the gun barrel. The procedure for carving grooves into the barrel of a gun is called rifling. Cutting the grooves leaves high parts, or lands, intact between them. The grooves grab the bullet as it traverses the barrel and cause it to spin and thereby increasing its accuracy of hitting the intended target. Old smoothebore rifles were not accurate beyond 100 feet or more, but present day rifled firearms are highly accurate to several thousands of yards.
Accuracy is not at the top of the list of the Calleigh Duquesnes (a character on CSI: Miami) of forensic firearms examiners. Their interest is how the lands and grooves of the rifling procedure mark the bullet.
When a gun barrel is manufactured, the rifling is etched inside of it. The depth of the grooves, the width of the lands, and the degree and direction of the spiral vary among different types of firearms and different manufacturers. These qualities help forensic examiners identify the type of gun that fired a bullet found at the crime scene and its manufacturer.
As an example, let us say a .32 caliber Smith & Wesson handgun has five lands and grooves with a right hand (clockwise) twist, and .32 caliber Colt has six lands and grooves with a left hand (counterclockwise) twist. Browning firearms also have six grooves, but have a clockwise twist. Marlin rifles utilize a method known as microgrooving. Microgrooving leaves between 8 and 24 narrow grooves within the barrel. Suppose a firearms examiner is given a .32 caliber bullet taken from an autopsy, and he discovers grooves compatible with a bullet having traveled down a barrel with five lands and a clockwise twist, the murder weapon was likely a Smith & Wesson, and forensic investigators can exclude all other handgun types and target .32 caliber Smith & Wesson handguns.
To make the firearms examiner's job easier, the Federal Bureau of Investigation (FBI) keeps a database known as the General Rifling Characteristics file to assist with making their determinations. It delineates the land, grooves, and twist qualities unique to known firearms. Similarly, bullet and shell casings can be matched with bullets and casings taken from other crime scenes that are listed in other databases.
Because smoothbore firearms like shotguns and older model firearms are not rifled, their bullets will not show any evidence of marking caused by lands, grooves, or twists. This makes the forensic firearms examiner's job a lot harder.
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