‘There was an effort to outlaw these’: NBC News correspondent explains AR-15 machine gun upgrade
NBC News justice correspondent Pete Williams shows a 'bump stock' and explains how it legally allows a semi-automatic firearm to mimic a machine gun.

MSNBC anchor Ali Velshi prodded NBC News justice correspondent Pete Williams to explain "the sheer amount of gunfire" used in the Las Vegas mass shooting that authorities say was committed by Stephen Paddock.

"Several rounds a second, that is something we not accustomed to civilians being able to do," Velshi noted of the massacre that has claimed 59 lives with another 527 injured.

Velshi is correct. In America, it is quite easy to purchase a semi-automatic firearm -- meaning each pull of the trigger completes all of the necessary mechanical steps for a bullet to be fired and another round chambered...awaiting a pull of the trigger to repeat the process.

This is different than what are known as fully automatic firearms -- machine guns that continue to fire and reload as the trigger is held down. Fully automatic firearms have been highly regulated since the Great Depression, when Tommy Gun robberies of banks resulted in a congressional ban.

What Pete Williams explained is that recent after-market upgrades have allowed easy-to-get semi-automatic firearms to function like highly-regulated fully automatic machine guns, effectively circumventing over eight decades of gun control.

"It is possible to put something on the weapon called a 'bump stock' and we know he bought two of these," Williams noted. "And we've been told that some of the weapons that were found in his hotel room had these 'bump stocks' attached.

Williams held up one example of a "bump stock" and demonstrated how it works.

Typically, fully automatic firearm functions by using the recoil force or gas of a fired bullet to automatically load the next road and fire it. Using a "bump stock" mimics this process with the stock of a rifle.

"Once you start it, it in essence pulls the trigger itself. "As a technical matter, the gun is still firing every time the trigger is pulled, but now the kick of the gun itself is pulling the trigger and so it's possible to get it going up to a very high rate of firing."

"Authorities say, that's how some of the rapid-fire shots that came out of those windows on the 32nd floor were fired," Williams noted.

The New York Times produced an audio interactive news story that isolates the rate of fire at the Mandalay Hotel in Las Vegas and compares it to the semi-automatic used in the Orlando nightclub massacre and the rate of fire of a fully automatic machine gun.

"Sheriff Joseph Lombardo of Clark County, Nev., said that at least 16 rifles, ranging from .308 to .223 caliber, and a handgun were retrieved from Mr. Paddock’s hotel room. A federal law enforcement official said that AR-15-style rifles were among them," The Times noted.

Williams has additional information on the weapons recovered inside the hotel room.

"They found the AR-15 type rifles, at least some of which had these 'bump stocks' attached and then they also found some high-powered sniper rifles that had the bipod legs on the front that allowed the rifle to be stabilized," Williams noted. "That's what gave him a great deal of the lethal firepower that he had."

"There was an effort to outlaw these in Congress about four years ago, but that effort failed," Williams noted.