Firearm identification is the process that deals with analyzing the bullets and the cartridge cases that are left behind at the scene of the crime to determine if they came from a particular firearm. Class and Individual Characteristics of Firearm decides if the bullet is fired from a suspected firearm.
The basis of firearm identification follows the principle of Locard’s principle of exchange. Firearm The surfaces of the firearm that come in contact with the softer cartridge case and bullet contain few marks common to a group of firearms and certain unique marks that make it different from the other firearms- even those of the same model.
Thus, class characteristics are characteristics that remain constant to a group of classes, while the individual characteristics are unique to a particular object.
Class Characteristics of Firearms
There are several general characteristics associated with the firearm that can help lead the investigation. These include the calibre of the barrel, the number and dimension of the grooved impressions inside the barrel, the direction of the twist of these rifling grooves, and different marks on the cartridge case.
1. Number and Twist of Lands and Grooves
it is the number of the lands and grooves protruding up and down in a helical pattern within the bore. Twist of lands and grooves in the direction in which helical pattern proceeds in the bore of a firearm that has been subjected to conventional rifling.
2. The Depth, Width, and Pitch of Lands and Grooves.
The distance between two lands in a bore is the width of the lands and grooves. The depth of lands and grooves is described as how deep is the raised portion of the barrel to the actual calibre of the firearm. The groove edge angle about the width and the steepness of the groove is the pitch of lands and grooves.
It refers to the spiral grooves cut into the barrel’s bore, which imparts a stabilizing spin to the projectile. Based on the direction, there are two types of rifling, namely leftwards rifling and rightward rifling. Based on the shape, there are six rifling patterns.
4. Caliber and Gauge.
In smoothbore firearms, the number of lead balls of a particular diameter it would take is equal to around known as gauge.
The distance between the opposite lands or grooves in the rifled firearm is known as calibre.
5. Firing Pin Marks
These are the marks or indentations created when the firearm’s firing pin strikes the primer of the centerfire cartridge case or the rim of the rimfire cartridge case.
6. Breech Face Marks
These are the marks developed when the projectile recoils rearward under the impact of high pressure and strikes with the breech face of the firearm.
7. Extractor and Ejector Marks
The striations formed by the extractor of the most autoloading or repeating firearm are called extractor marks.
The striations developed from the ejector present as an opposing edge on the rear end during the removal of the cartridge case from the chamber are known as ejector marks.
Individual Characteristics of Firearms
The individual characteristics can be marks produced by random imperfections during manufacturing or irregularities caused due to use, corrosion or damage.
Individual characteristics are unique to a firearm and distinguish each firearm from the other firearm. The individual characteristics of a firearm are the individual characteristics found on a cartridge case and bullet.
These are the random marks produced incidentally during the manufacturing of a firearm by the tools and machinery.
These are imparted inside a bore of a firearm during rifling. These are the irregularities found on the interior surface of the barrel that is produced due to wear and tear, corrosion or damage.
The striations present in the lands and grooves of a weapon link directly to the rifling process and link to the manufacturer of the weapon. The lands and grooves from a fired bullet help it link to the bore of the firearm.
These individual and class characteristics of firearm are examined and compared using comparison microscopy and stereoscopy.
To determine whether or not a particular gun has fired the questioned bullet, a detailed comparison is made of the markings on the questioned bullet with corresponding markings on the test bullets fired through the suspected gun.