Semi-automatic: Almost all guns in the _______ today, including pistols, rifles, and handguns, are semi-automatic. A semi-automatic firearm fires a single bullet each time the trigger is pulled. The weapon can be loaded with magazines containing various numbers of bullets; the semi-automatic handgun used in the Fort Hood shooting, for instance, was equipped to shoot 20 rounds in 5.3 seconds. Different grips and mounts can modify the ease and accuracy with which a semi-automatic gun can be fired; ________ attempted to limit such add-ons in the 1994 ban.*

AR-15: The semi-automatic rifle Adam Lanza used as his primary weapon. Lanza’s specific model was a Bushmaster M4 Type Carbine, using several magazines of 30 bullets each. His mother, Nancy Lanza, acquired the gun legally. Certain models were proscribed under the 2004 ban, but myriad loopholes allowed slightly modified versions of the gun to be sold legally. In the Aurora shooting, James Holmes used an AR-15 which he had purchased legally from gun stores and ammunition that he had purchased on the Internet.*

Semiautomatic pistol

This is a more recent development than the revolver, originating late in the 19th century, mostly through the efforts of John Browning. In fact, almost every semiautomatic handgun available today is a copy of his two most famous designs: the Colt model 1911A government 45 and the Browning Hi Power 9 mm. The anatomy of a semiautomatic pistol is given below:

The advantage of semiautomatics is the use of recoil generated by the fired cartridge to eject the empty cartridge case, load the next cartridge, and cock the hammer. This is more conducive to firing multiple shots, so many are designed to carry 15 to 19 rounds. Disadvantages include a more complicated mechanism, require more practice to use, and cartridge cases must be short to work well. Revolver cartridges are more powerful than semiautomatic cartridges for this last reason.

The barrel is normally hidden by the slide. Choices of barrel length are limited. The slide is a key part to the operation of a semiautomatic:

The slide is able to move back along the axis of the barrel under tension from a spring. Since the cartridge base rests on the slide, the slide does just that under the force of recoil generated by the firing of the cartridge. As the slide and empty cartridge case are accelerating backwards, the case is struck by a stationary piece of metal that bumps it to the side. This is conveniently located next to a hole in the slide, so that the empty cartridge case continues its acceleration in a direction perpendicular to the pistol and into the air, landing from 2 to 20 feet from the fired gun. The rearward- moving slide also cocks the hammer. After the case is clear the slide hits a stop and the spring tension starts it forward. The magazine spring is pushing on a column of rounds tight up against the bottom of the slide. As the slide comes back by the column of cartridges, it grabs the top one and pushes it forward and up a short ramp into the chamber where the slide locks it in place. The radiograph below demonstrates the location of the spring in relation to the slide and the barrel.

The handle, or butt, is more important here because it contains the magazine holding the cartridges. Safety mechanisms prevent accidental firing. Some lock the hammer, while other designs lock the trigger. In the radiograph below, a loaded magazine is present in the handle.

Even on open ground ejected cases may be difficult to find, as they typically roll into a hiding place such as grass or small depressions in the ground. Thus, ejected cases will virtually always be left behind at the scene, but must be searched for diligently.