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