"I want the day to come where we don't have to have this ridiculous debate time and again in America," he said in frustration. "I just cannot have this debate anymore. It is ridiculous."
Efficient but deadly
The AR-15 is an efficient killing machine that originated as a U.S. Army rifle, military documents say. At the time, it was a fully automatic weapon.
The Army tested it as early as 1958.
It was said to be more effective than its predecessor, the M14, and showed distinct advantages against the AK-47, the weapon carried by the United States' opponent in the Vietnam War, the Viet Cong.
It led to a surge in military purchases during the mid-1960s. Eventually the AR-15 was further developed into the M16 rifle.
The original M16 patent ran out years ago, and now the AR-15 is manufactured by several gun makers.
The AR-15 has since become a semiautomatic rifle, firing one bullet per squeeze of the trigger. But like the M16, ammunition is loaded through a magazine.
In the Sandy Hook Elementary School shooting, police say Lanza's rifle used numerous 30-round magazines.
An AR-15 is usually capable of firing a rate of 45 rounds per minute in semi-automatic mode.
Under the 1994 federal ban on such weapons, buying some variants of new AR-15s was against the law. The ban expired in 2004.
It's easy to get an AR-15. They can be purchased online or in a gun store. Online purchases are sent to a gun store for pick-up and a background check is still required.
Early Tuesday, one online retailer had a special running: Plunk down $999, and it can be delivered to the gun retailer of your choice in 10 days.
The gunfire in the nation's capital quickly drew echoes from Washington politicians.
Some, like Feinstein, renewed their calls for restrictions on semi-automatic weapons.
The California Democrat, one of the strongest proponents of a ban on assault rifles, called for Congressional action. "We must do more to stop this endless loss of life," she said.
The new mass shooting is nothing new, bemoaned Democratic Sen. Jay Rockefeller of West Virginia. "We are becoming far too familiar with senseless, tragic violence. This is the seventh shooting since 2009, and these repeated incidents demand our attention," he said.
But their words will likely fade away, said Democratic Sen. Joe Manchin of West Virginia. The broad support for gun ownership in Congress is too strong.
He doesn't think the shooting will be enough to sway the 60 votes a gun control measure would need to break a Republican filibuster.