As hell rained down on innocent concertgoers in Las Vegas, trained law enforcement officers were almost positive the gun blasts coming from the gunman's perch at the hotel had to be automatic fire. As it turns out, they were mostly wrong.

Federal agents now have determined that twelve of the AR-15 rifles found in Stephen Paddock's hotel room were modified, upgraded to mimic a fully-automatic weapon by a device that anyone can buy, over the internet or at a sporting goods store.

As a result, the country on Wednesday seemed fascinated by the device called the "bump stock" or the "Slide Fire."

The feds say it's what Paddock used to mimic a machine gun and killed 59 and injured more than 500 people.

It's a totally legal, modification device you can put on your AR-15. Instead of having to rapidly pull the trigger, with the "bump stock," you can just hold the trigger and the recoil action turns your totally legal semi-automatic into a fully automatic.

How easy is it to buy one? WFAA found one at a Cabella's Sporting Goods store in Fort Worth for $300.

They can be found online for around $100. We were in and out of the store with our own "Slide Fire" in about 12 minutes.

Shooting instructor Travis Bond of DFW Shooters Academy agreed to install and test it for us. Despite Bond's significant experience and knowledge of weapons, trying to decipher the instructions and navigate the pieces proved challenging to him.

We then traveled to the shooting range to test the firing speed of the newly converted AR-15. Bond had never fired a "bump stock" before. Even before we got started, he shared his thoughts.

"I think basically the "bump stock" is more of a toy," said Bond. "I think it's like Tannerite or some of those other weapon-related novelties. If used properly, they are safe."

But safety is compromised if the device does not work correctly. After several attempts to get the "Slide Fire" to work, the trigger alone worked and the weapon fired, but our $300 product appeared to malfunction. So Bond pulled out his own semi-automatic, AR-15 to prove a point.

He began rapid firing the weapon as designed. So many rounds came out, the barrel of the gun started smoking.

"It's not quite as fast but what more do you need than that?" Bond asked.

His final analysis? With or without the "bump stock" or "Slide Fire," a gun in the wrong hands can spell tragedy.

"Statistically would there have been a difference if Paddock had used a semi-automatic?" asked Bond. "I don't know and I don't have any way of knowing. But I can tell you this, the damage would have been significant either way."

Some conservative Congressmen, including two Texans, Sen. John Cornyn and Rep. Bill Flores, were actually discussing the prospect of banning the "bump stock" style modification device.

California Senator Dianne Feinstein introduced legislation on Wednesday to ban the weapon modification devices.