An assault rifle is a military firearm that is chambered for ammunition of reduced size or propellant charge and has the capacity to switch between semi-automatic and fully automatic fire. It uses an intermediate-rifle cartridge and a detachable box magazine. The U.S. Army defines assault rifles as "short, compact, selective-fire weapons that fire a cartridge intermediate in power between submachine gun and rifle cartridges." In this strict definition, a firearm must have at least the following characteristics to be considered an assault rifle:

• It must be capable of selective fire.
• It must have an intermediate-power cartridge.
• It must have a detachable magazine.

However, in the gun community, an “assault rifle” generally refers to a military weapon that is fully automatic. In the United States, selective-fire rifles are legally defined as "machine guns," and civilian ownership of those has been tightly regulated since 1934 under the National Firearms Act and since 1986 under the Firearm Owners Protection Act. The term "assault weapon" is often used interchangeably with "assault rifle," but this use has been described as incorrect and a misapplication of the term.