Rifle, a shoulder weapon having grooves (called rifling) inside its barrel to improve accuracy; or an artillery piece or naval gun with a grooved barrel. The grooves are cut spirally to give the bullet or shell a rotary motion. This improves accuracy because spinning tends to cause a moving object to maintain a straight course. Shotguns and other firearms with smoothbore (unrifled) barrels are not classified as rifles.
A carbine was originally a short rifle for use by mounted troops, but the term was later applied to any short, lightweight rifle. An automatic rifle is similar to a machine gun; the principal distinction is weight and accuracy—an automatic rifle is lighter and, when firing in the automatic mode, less accurate. An assault rifle is a lightweight automatic rifle firing a cartridge with somewhat less power than the type of cartridge used by a full-size military rifle. The submachine gun is similar to an assault rifle but has a shorter range and fires a larger bullet.
In the United States, it is illegal to sell automatic and certain types of semiautomatic rifles to the general public.