Expert discusses 'bump stock' modification to rifles

Not much is known about motives of the alleged gunman in the worst mass shooting in modern U.S. history, but who he was and how he pulled off the massacre is becoming more clear.

Stephen Paddock was retired, known as a professional gambler, and worked as a letter carrier and IRS agent in the 1970s and 80s.

Investigators said he had 23 guns in his room at the Mandalay Bay Hotel and Casino and he put a camera in a food service cart outside his hotel room. He packed them all in ten suitcases.

Authorities also said he had at least one device that made the guns fire bullets in rapid succession called a "bump stock."

Also known as a "slide rifle," a "bump stock" is an attachment which can increase the rate at which a rifle can fire rounds. "Bump-firing" is a technique whereby a shooter pulls a firearm forward into a stationary trigger finger. The recoil from firing sends the firearm rearward while the forward pull brings it back into contact with the trigger finger thereby firing the rifle again. "Bump stocks" effectively replace a gun's standard stock, with a mechanism that aids in allowing the rifle to slide back and forth behind a stable trigger finger.

Technically, that means the trigger is being pulled once for each round fired, keeping the weapon a legal semi-automatic.

It costs under $300. That was the price before the Las Vegas incident and it requires nothing special to make the purchase.

FOX News and the Associated Press contributed to this report