According to the textbook "Crucial Elements of Police Firearm Training," a semi-automatic firearm has a mechanism that automates most of the process of shooting. It automatically loads ammunition from an internal or external magazine into the firing chamber, extracts and ejects the spent cartridge when a shot is fired, and then uses some of the energy of the fired shot to load another cartridge from the magazine so that the shooter can fire again. This enables a shooter to fire a succession of shots quickly, as long as he or she squeezes the trigger again each time [source: Johnson].
The automation of the loading, firing and reloading process, and the utilization of energy from one round to put the next one in firing position, differentiates a semi-automatic from firearms such as bolt-action rifles and revolvers. In turn, the requirement that a shooter repeatedly depress the trigger again for each shot differentiates a semi-automatic from a fully automatic weapon such as a machine gun. That sort of weapon will continue to fire, as long as the shooter's finger is on the trigger, until it runs out of ammunition [source: Johnson].
Gun rights advocates often say that semi-automatic firearms should not be considered "assault weapons" because they are not fully capable of "spraying" automatically. But the 2008 edition of "Gun Digest Buyer's Guide to Assault Weapons," a book for gun enthusiasts, lays out a pretty specific definition: Semi-automatic rifles that accept detachable magazines holding more than 10 rounds of ammunition, and often include features -- such as a pistol grip, flash suppressor, folding stock or bayonet mount -- and are designed to make them cosmetically resemble fully-automatic military weapons. The first such weapon was the Colt AR-15, a semi-automatic rifle clone of the military's M16, introduced to the civilian market in 1964 [source: Peterson].
Assault weapons first earned an unsavory reputation in part because of events such as the 1989 Stockton, Calif., school massacre, in which a mentally unstable drifter shot five children to death with an AKM-47, a semi-automatic copy of a Soviet-Bloc military rifle [source: Associated Press].