Automatic Firearms

Automatic firearms are weapons that are designed to fire as long as the trigger is held down. These weapons are usually of the true assault weapon variety—or larger—such as machine guns. They are illegal in the United States except under special permits from the BATF, although some people hold these weapons illegally. At crime scenes, they will leave behind the same projectiles and cartridge cases as semiautomatic weapons.

Long guns are designed to be fired using the shoulder for support. There are two main types of long guns seen by investigators: rifles and shotguns. The main difference is that shotguns do not have rifling in the barrel. This means that fired projectiles from a shotgun cannot be compared to test fires from a particular weapon. Ammunition for a shotgun is a shotshell. It usually contains buckshot but can contain a single projectile known as a slug. Once fired, it is a shotshell case or casing. Rifles, on the other hand, have rifling in the barrel, and their projectiles can be used for comparison. The function of these weapons is very similar to that of the handguns. Rifles and pistols are characterized by the diameter of their barrels. In rifled barrels, the diameter is measured land-to-land. This feature is called caliber. Caliber can be stated in inches or millimeters. It is sometimes stated in what might be called firearms slang.