When a Las Vegas SWAT team breached Stephen Paddock's hideout on the 32nd floor of the Mandalay Bay Resort and Casino, they found the gunman dead on the hotel room's beige carpet surrounded by more than a dozen rifles and what had to be nearly a thousand spent bullet cartridges. He killed 59 people and wounded more than 500 others.

At least two of the weapons were equipped with so-called bump stocks, attachments that allow a semi-automatic rifle to fire bullets almost as fast as a fully-automatic rifle. Legal, comparatively affordable, and now apparently deadly, these accessories are front and center in the gun-control debate.

EXCLUSIVE: 2 of the 23 guns found in #LasVegas suspect's Mandalay Bay hotel room. Photos obtained by @JacquiHeinrich https://t.co/PX0w8X167U pic.twitter.com/wwnc8MWdPO — Boston 25 News (@boston25) October 3, 2017

So what is bump-firing stock exactly, why is it legal, and why would anyone want one?

What is bump-firing: A technique that uses the recoil of a semi-automatic firearm to shoot multiple rounds. The editors of Guns America describe the technique as holding "the gun in a loose way and allow it to rock back and forth against the trigger finger, which simulates automatic fire." As they note, as long as there have been semi-automatic guns—both pistols and rifles—people have been bump-firing them.

YouTube is full of perhaps less than wise outdoorsmen using the "belt-loop method" to burn through multiple rounds in only a few seconds. But like fanning the trigger of a revolver, bump-firing can be especially inaccurate.

Enter the bump-fire stock: While semi-automatic rifles can only fire one round with a single trigger pull, fully-automatic rifles fire multiple rounds in quick succession. A bump-fire stock bridges that gap without losing significant accuracy.

The accessory replaces the normal fixed stock with a sliding shoulder rest and a "trigger step." A shooter can achieve something close to automatic fire by "pulling apart" the rifle.

A bump-firing stock "allows you to shoot as fast as you want to", explains Jeremiah Cottle, inventor of the name brand Slide Fire stock. "You hold your finger on the trigger rest and push forward to fire the gun," Cottle said in 2011 interview, adding that, while not technically automatic, the device makes it possible to "shoot one round, two rounds, three rounds, 15 rounds or a full magazine."

Again, YouTube provides a decent tutorial.

Out of the box, do bump-fire stocks turn AR-15s into machine guns? Sort of. The accessory certainly increases rate of fire out of the box. But the stock can wear out the mechanics of a gun, like snapping the firing pin that was designed for semi-automatic fire.

And while the accuracy is comparable, it's not identical. Pulling the gun apart, so to speak, is more difficult than pulling the trigger of a machine gun.

What's the selling point? Anyone who has ever fired anything at full-auto knows the appeal. A while back Vice News sent one of their reporters to the Big Sandy machine gun shoot in the Arizona desert. Even that liberal journalist got trigger happy after sending round after round down range.

But because of the 1986 Firearm Owners Protection Act, legal machine guns are rare, exceptionally expensive, and extremely difficult to own (learn more here). Anyone who wants to go fully-automatic must undergo an extensive background check and register with the Bureau of Alcohol, Tobacco, Firearms, and Explosives, and pony up tens of thousands of dollars.

A bump stock provides a poor man's alternative. Models run anywhere from $99 to $400 at sporting good stores everywhere and can be installed in less than an hour. No gunsmith required. At a fraction of the cost and without the federal registration, a person can jerry-rig a machine gun.

How is this legal? While federal law prohibits the sale of any machine guns manufactured after 1986 and it heavily regulates conversion of semi-automatic into fully-automatic rifles, a bump-firing stock only achieves nearly full-automatic firepower. It does not achieve the legal definition of a machine gun.

The guts of the gun are not modified to fire multiple rounds. The only thing that changes is the stock, which "bumps" against the shooter's finger, simulating full-auto. Cottle says that if the ATF wants to ban bump-firing stocks "they basically have to modify the definition of a machine gun."

His product comes with a letter from the ATF certifying its legality and, in a 2011 interview, Cottle explained that his product has "no moving parts," adding that "the only thing firing the gun is you."

Aside from a few states that have banned them, bump-fire stocks are completely legal.

Will they stay legal? While it's not clear if Paddock used the bump-fire guns or a truly fully-automatic machine gun, the attack has inspired increased scrutiny. Sen. Dianne Feinstein, D-Calif., tried banning the stocks after the Newtown, Conn., shooting in 2013.

"Replacement bump-fire stocks allow semiautomatic weapons to reach rates of fire approaching those of fully automatic weapons," Feinstein wrote in the Orange County Registrar. Citing California law enforcement, the Democrat argued that these "military features" are suited "for the battlefield – not our streets."

The argument that failed to convince Republicans in Congress four years ago could win them over today. President Trump Tuesday said that lawmakers will "be talking about gun laws as time goes by." And considering the alternatives, banning bump stocks could be an ideal reform that even the National Rifle Association might support.

After President Reagan signed the 1986 law prohibiting the sale of new machine guns, the NRA has gotten onboard with machinegun regulations. After the Las Vegas attack, they might see a ban on bump-firing stocks as a logical extension and a politically feasible solution.

Should this stay legal? Greg Kinman, a retired police officer who runs the viral Hickok45 YouTube channel, seems to believe they should. His 2012 review of the Slide Fire Stock installed on a Colt M4 Carbine, showing Kinman destroying clay pots and vaporizing Faygo pop bottles, has nearly 900,000 views.

In a Facebook post after the Las Vegas attack, Kinman slammed the media for thinking semi-automatic and fully-automatic guns are "like nuclear weapons." But he added that the "whacko could have carried seven or eight fully loaded 1860 Henry lever guns (Civil War era technology) up to that room and created a similar massacre."

Philip Wegmann is a commentary writer for the Washington Examiner.