The Gun Glossary

What’s a semi-automatic? What counts as an assault weapon?

A common type of handgun is a semi-automatic pistol, a small firearm with a single, nonrevolving chamber

Photograph by Scott Olson/Getty Images.

Read the rest of Slate’s coverage of the Sandy Hook school shooting.

Following Friday’s tragic mass shooting at Sandy Hook Elementary School in Newtown, Conn., semi-automatic guns and assault weapons have been thrust into the public spotlight. Politicians are discussing the merits of a renewed assault weapon bans, while police and reporters are investigating the weapons used by Adam Lanza on his rampage. But the terms used by the media are often confusing and imprecise, and few reporters explain the differences among various types of firearms. Below is a glossary of gun terms that are relevant in the wake of the Sandy Hook shooting.

Assault weapons: There is little agreement on what constitutes an assault weapon, even among firearm enthusiasts. Legal definitions vary from state to state: Connecticut defines an assault weapon as “any selective-fire firearm capable of fully automatic, semi-automatic, or burst fire at the options of the user.” In Virginia, any gun with a magazine capacity greater than 20 rounds counts. The now-expired Federal Assault Weapon Ban, passed in 1994, laid out complex guidelines to define which firearms were considered assault weapons. (Most were AR-15s and AK-47s.) The federal definition of an assault weapon expired with that law in 2004.

However, most gun experts agree that fully automatic firearms (defined below) count as assault weapons, as do some higher-caliber semi-automatic guns.

Semi-automatic: Almost all guns in the United States 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; Congress attempted to limit such add-ons in the 1994 ban.*

Automatic: An automatic weapon fires bullets as long as its trigger is squeezed. Well-known examples include machine guns, which are automatic firearms often mounted to a wall or rail in order to increase stability and accuracy, and submachine guns, which are portable.* Automatic weapons vary in legality across the United States; they are regulated by the federal government but are legal to own.

Handgun: The only universally agreed-upon definition of a handgun is a gun that can be held it in a single hand. According to a gun expert at NOVA Firearms, a firearms dealer in Northern Virginia, no handguns would ever be considered an assault weapon. Most are semi-automatic, but some handguns do qualify as automatic weapons, able to shoot many rounds at once. A common type of handgun is a semi-automatic pistol, a small firearm with a single, nonrevolving chamber.* The gun Jared Lee Loughner used to shoot Gabrielle Giffords and kill six people in Tucson, Ariz., a Glock 19, was a semi-automatic pistol.  

Revolver: A firearm with a revolving cylinder that can fire several bullets (usually six) at a time without reloading. Each time the hammer is cocked, the cylinder moves the next bullet into place. Most remaining “cock and fire” guns—those for which the gun must be cocked each time it is fired—are revolvers. Cock and fire revolvers are called “single action” revolvers. No revolvers are fully automatic.*

Rifle: Long-barreled firearms with a groove carved into the barrel walls to give bullets spin. Rifles can be single action, semi-automatic, or fully automatic.*

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.*

*Corrections, Dec. 18, 2012: This article originally stated that grips and mounts can modify the power of a semi-automatic gun (they modify the ease of use); that machine guns are mounted to a wall or rail (they often are, but not always); that a pistol is a firearm with a single, nonrevolving chamber (some pistols are, but not all); that no revolvers are semi-automatic (none are fully automatic); and that James Holmes bought his weapons on the Internet (he bought his ammunition on the Internet). The article also defined a rifle as a firearm with grooves on its barrel walls. Many other modern firearms, including handguns, have rifled barrels; more precisely, a rifle is a long-barreled gun with such grooves.