An examiner relies on two types of characteristics when performing comparisons, class and individual characteristics. Class characteristics are common to a group of firearms and are determined by the manufacturer. Class characteristics include caliber, number of lands and grooves, direction of twist of the lands and grooves and the widths of the lands and grooves. Individual characteristics are the markings that are unique to a particular firearm and allow an examiner to say that a projectile or cartridge case was fired in a specific firearm.
