An assault rifle is a select fire action with a reduced power cartridge to make firing in full auto easier. Select fire implies the ability to fire full auto or 3-shot multi-burst OR semi-auto. The M14 fired .308 in full auto mode. The cartridge was too powerful to control accurately in full auto. The AK-47 fires a .30 caliber round too, but uses less powder. It is easily controlled in full auto. The M16 uses the Winchester .223. It is also a reduced power cartridge. In a select fire rifle, it can correctly be labeled an "assault rifle". The semi-auto only AR-15 is NOT an assault rifle. It only fires one round per trigger pull. The anti-gun liars take advantage of the physical similarity of the M16 and AR-15 rifles and show videos of an M16 being fired full-auto while claiming that rifle can be purchased at the local gun show.
Also, detachable magazines permit rapid reloading. And the intermediate power cartridge allows the user to carry more ammo, as well as control the weapon.
True, by DoD definition. However an "assault weapon" is whatever gun some gun grabbing politician or LEO wants to ban. Right now there is no *federal* definition of "assault weapon" as the definition expired with the ban.