Class characteristics are also very important because of their ability to narrow the focus of a case. Class characteristics in firearms cases are determined by the manufacturer. They can include the number and width of lands and grooves in the barrel of a gun, the direction of twist, and the caliber of the gun. The firearms analyst can quickly include or exclude a particular weapon as having fired a bullet through comparison of these manufactured characteristics. A cartridge is an unfired case containing a bullet. Once it has been fired in a weapon, it is a cartridge case, or cartridge casing. Cartridge casings can also be examined using class characteristics to identify the caliber and features of the weapon for which they are intended. There are center-fire and rim-fire cartridges, and cartridge cases from semiautomatic weapons will have extractor and/or ejector markings. (The exception to these analyses is shotgun ammunition, as will be discussed later). With firearms identifica- tion, individual characteristics are specific to a particular weapon.