It is both legal and lethal, seven pounds of steel and plastic that can fire a bullet at roughly 3,000 feet-per-second. To some, the AR-15 is a brilliant piece of engineering and a symbol of one of America’s most basic freedoms. To others, it is an obscenity, an assault weapon with no justifiable place in civilian hands. For both sides, the rifle used in the Newtown and Aurora shootings has become a lightning rod in a wrenching debate over what a gun like this is for and whether anyone should have one. CNBC’s Brian Sullivan examines the controversy and the rise of the AR-15, the rock star of America’s gun industry.