These are class characteristics and individual characteristics. Class characteristics are measurable features of a specimen that indicate a restricted group source. On bullets the class characteristics of interest to the Firearm Examiner are the rifling specifications of the barrel from which the bullet was fired. These include caliber, number of lands and grooves, direction of twist of the lands and groves, and widths of the lands and grooves. If an evidence bullet and test bullets fired from a suspect firearm have the same class characteristics, the Firearm Examiner can conclude that the evidence bullet could have been fired from the suspect firearm.
Individual characteristics are marks unique only to that particular firearm barrel.
In a barrel the individual characteristics are produced by the random imperfections and irregularities of the tool or tools used to produce the lands and grooves and by use, corrosion or damage. If an evidence bullet has the same class characteristics and matching individual characteristics to test bullets fired from a suspect firearm, the Firearm Examiner can conclude that the bullet was fired from the suspect firearm. A Firearm Examiner must also be skilled in firearm function . An examiner needs to know and understand the various types of firearms and the types of actions employed by the firearms manufacturers in producing these firearms. A Firearm Examiner must be able to describe to a jury how a particular firearm functions and why a particular firearm may have malfunctioned.