Bullet ID Lab

Macroscope mag 5-40X

Water tank to obtain the standards for the bullet

Bullets collected for comparison to a specific firearm are examined first to see if they are of a caliber that could have been fired from the submitted firearm. 

They are then examined to determine if the pattern of rifling impressions found on the bullet match the pattern of rifling contained in the barrel of the questioned firearm. 

Striations have the potential to be consistently reproduced in a unique pattern on every bullet that passes down the barrel of a firearm. photo on the right shows

split screen imageScientists see using a comparison microscope.

The right side shows the test bullet fired from the suspect's gun into the water tank

The left side, shows the bullet recovered from the crime scene. Marks or striations on each bullet match,indicating that the two bullets were fired from the same weapon.

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One of the biggest problems in making an identification is that few evidence bullets are submitted intact.  Most are badly distorted, wiped and/or fragmented. 

Caliber indicates the diameter of a bullet in hundredths of an inch.  A bullet that is 30 hundredths of an inch (.30) in diameter is called a 30 caliber bullet.The caliber of the bullet is just the first class characteristic that must agree with the questioned firearm

Rifling: grooves cut or formed in a spiral nature, lengthwise down the barrel of a firearm, and 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.

Firearm examiners 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.

raised portions of the rifling are known as lands and the recessed portions are known as grooves.

When a weapon is fired, these lands and grooves cut into the bullet, putting spin on it as it travels through the barrel of a firearm. Because bullets have an oblong shape, spin is necessary for accurate flight.

A bullet is slightly larger in diameter than the bore diameter of the barrel in which it is designed to be fired. 

The bore diameter is the distance from one land to the opposite land in a barrel.  As a result, a rifled barrel will impress a negative impression of itself on the sides of the bullet like those seen below.  

Breech face markings – These come from the area surrounding the firing pin of the gun. After the cartridge powder is ignited by the firing pin striking the primer cup, tremendous pressure is exerted in the chamber of the weapon, forcing the back of the cartridge case against the breech face of the weapon.

Extractor marks - After a semi-automatic pistol has been fired, an extractor pulls the cartridge from the chamber and ejects it from the pistol.

Other marks on bulletsFiring pin impressions - When a weapon's trigger is pulled, the firing pin is forced into the primer cup located at the rear center of the cartridge.

http://www.youtube.com/watch?v=e8HR5TxlPMc&NR=1&feature=fvwp

http://www.youtube.com/watch?v=WGHCTMTh7TE