How bump stocks turn semiautomatic rifles into near-automatic weapons

— -- Half of the 23-gun arsenal discovered inside Stephen Paddock's 32nd floor room at the Mandalay Bay Resort & Casino were rigged with bump stocks, devices that modify rifles to fire faster, repetitive rounds, mimicking a fully automatic machine gun.

From 10:08 p.m. to 10:19 p.m. Paddock's gunfire volley bursts caused unspeakable bloodshed among concertgoers below.

He killed 58 people and injured over 500 before security guards and police stormed his room and he took his own life, Las Vegas authorities said.

The device rigged to some of his weapons can be bought legally for $99 to $299 and manipulates the rifle's stock so that when a trigger is pulled the recoil is seamless, giving the shooter the ability to pull a trigger and fire bullets with almost imperceptible stoppage.

Bump stocks have been on the market for almost a decade, experts told ABC News, and while the debate gathers steam over whether these devices should be banned, for now, they remain legal and can be acquired with relative ease.

Some members of Congress have already begun an effort to stop the sales of bump stocks and similar devices. Sen. Diane Feinstein introduced a bill on Wednesday that would make it unlawful to sell or trade devices that are designed to "accelerate the rate of fire of a semiautomatic rifle but not convert the semiautomatic rifle into a machine gun."

Currently, bump stocks can be bought and used in most states, giving gun owners a free-flow motion that's the closest thing to a machine gun firing experience.

While bump stocks simulate the same action of an automatic firearm, they are not fully automatic and so have not been banned by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF).

An automatic weapon is defined by the ATF as "Any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot without manual reloading, by a single function of the trigger.”

In a letter dated June 7, 2010, the ATF said the bump stock "has no automatically functioning mechanical parts or springs and performs no automatic mechanical function when installed."

At a press conference on Tuesday, Jill Snyder, special agent in charge of the ATF's San Francisco Field Division, said the classification "of these devices depends on whether they mechanically alter the function of the firearm to fire fully automatic."

While it's too early to tell if Paddock's weaponry met that standard, the bump stock changes the way the gun is held and fired and is known to increase the rapidity of fire.

Besides being readily available to purchase, there are how-to videos online showing ways to trick out a semiautomatic weapon with a bump stock.

But while bump stocks up the ante in terms of firepower, experts told ABC News they sacrifice accuracy.

"From a personal experience, it is very difficult to maneuver," Anthony Cooper, owner of Maryland-based Gun Monkey's Armory. "The recoil bumps your finger on the trigger, mimicking fully automatic."

The 35-year-old Navy veteran, who served as a gunner's mate, third class, and deployed to the Persian Gulf and East Africa, told ABC News that when bump firing your grip on the gun changes from the conventional rear to "up front by the hand guard."

It's usually shot from the hip, he said, but firing with a bump stock isn't easy.

"There's a lot more kick and a lot more recoil," Cooper said. "It takes a strong person to be able to bump fire a gun."

The device, which already is seamlessly creating mini explosions, expends more heat and smoke and makes its harder to hold steady and therefore easier for the shooter to miss.

"They tend to rise on you," said Hector Tarango, retired ATF resident agent in charge of the Fort Worth Field Office. "The gun goes up with inertia.

"These weapons will have a forward vertical grip which allows the shooter to pull down on the weapon and you do lose some accuracy."

Tarango said the high-end bump stocks are all made of composite plastic but engineered better so they're "more reliable" and slide with "less friction."

Tarango called them a workaround for getting near-automatic firing power without violating the current law.

"For a little money they were able to replicate a machine gun without messing with the definition in the books," he said. "They didn't change the trigger or the internal parts."