What is an assault rifle?

Some gun stores say that an assault rifle is a rifle that shoots fully automatic, have features like pistol grip, or a flash hider. Also some say that versions of semi-automatic are available for civilian use should be called tactical  rifle or modern sporting gun. But if Obama wants to get rid of guns, that's no fair because Obama's has security guards that carry guns and there not going to ban guns on national security.

Guns to groceries: Villaraigosa's buyback program good P.R., bad policy - LosAngeles Times