Forensic examination of a firearm is the process of examining the various characteristics of firearms, bullets, and cartridges left behind at the crime scene.
Specialists in this field are often tasked with linking bullets and cartridges to the weapons and the weapons to the individuals. The various characteristics such as breech face marks, firing pin marks, extractor & ejector marks, size of the bullets, striations found on bullets are compared with the test-fired cartridges and bullets.
Comparison is made simultaneously between the questioned samples and the test-fired samples using a comparison microscope.
Cartridge Case Examination
The examination of cartridge cases depends on the unique marks left by various parts of the firearms that include the following:
- Breech Face Marks:- These marks come from the area surrounding the firing pin of the gun. After the propellant powder is ignited by the firing pin striking the primer cup, tremendous pressure is exerted in the chamber of the weapon that forces the back of the chamber of the cartridge case against the breech face of the weapon.
- Firing Pin Impression:- As the firing pin strikes the primer cap with force, there will be marks of the firing pin on its surface. These marks are due to imperfections, finishing marks, etc. they are different for different firearms and are rarely or never duplicated.
- Marks from Extractors and Ejectors:- all firearms have some form of extractor or ejector. The extractor marks are not found on the pistol cartridge cases. The pistols have special ejectors, which are likely to leave marks on cartridges cases fired in them. Repeater and automatic arms usually impress definite and recognizable marks like size, shape, and location on the rim of the cartridge cases. Sometimes their sliding marks offer more identification.
- Marks Due to Expansion:- At the time of firing, the cartridge cases expand and may take up marks or certain irregularities, which may occur in the region where the barrel, breechblock, and extractor meet.
Bullets fired through the rifled firearms receive both the class and individual characteristics of the barrel from which they are fired. The bullet will show primary markings left by the gun barrel’s lands and grooves and reveal the fine striations in all the marks. These are the imprint of the minor irregularities in the barrel and are never duplicated by the different firearms.
To determine whether or not a particular gun has fired the questioned bullet, a detailed comparison is made of markings on the questioned bullet with the corresponding marks made on the test bullets fired through the suspected gun.
When the bullet seized from the crime scene is greatly deformed, or when only fragments are recovered, comparative chemical analysis of the questioned and the known bullets by spectrographic analysis may yield helpful information.
The comparison of the cartridges and the bullets can also be made using the firearms ID. Software that is designed to produce virtual comparison and helps match the questioned sample and the test samples.
The foreign material in the form of blood, tissues, etc., on the fired bullets can often be beneficial to link it to the targets.