The investigation into America's deadliest shooting quickly revealed that Stephen Paddock had modified some of his weapons with bump stock devices before opening fire on the crowd at a Las Vegas country music concert.

The 64-year-old killed 58 people and injured 489 before turning one of his weapons on himself.

Since then the bump stock has come under scrutiny.Both the National Rifle Association, usually staunch opponents of gun restrictions and the Republican Party have signalled a willingness to crack down on the devices.

The Walmart superstore chain has also pulled the devices from its online marketplace, saying they should never have been offered there and attributing the sales to a third party vendor.

But what is a bump stock?

In essence, they convert semiautomatic weapons into automatic weapons, which are mostly banned in the US (automatic guns purchased before Congress enacted the prohibition in 1986 are allowed).

A semi-automatic weapon fires a single bullet with each pull of the trigger, whereas an automatic fires for as long as you compress the trigger.

Simple physics allow the bump stock, which is fitted by the trigger to minimise that distinction.

Las Vegas shooting – in pictures Show all 15 1 /15 Las Vegas shooting – in pictures Las Vegas shooting – in pictures People scramble for shelter at the Route 91 Harvest country music festival after gun fire was heard Getty Las Vegas shooting – in pictures People carry a person at the Route 91 Harvest country music festival after shots were fired David Becker/Getty Las Vegas shooting – in pictures People run from the Route 91 Harvest country music festival after gun fire was heard David Becker/Getty Las Vegas shooting – in pictures A handout photo released via Twitter by Eiki Hrafnsson (@EirikurH) showing concertgoers running away from the scene (C) after shots range out at the Route 91 Harvest festival on Las Vegas Boulevard EPA/Eiki Hrafnsson Las Vegas shooting – in pictures People lie on the ground at the Route 91 Harvest country music festival after hearing gun fire Getty Las Vegas shooting – in pictures A man in a wheelchair is taken away from the Route 91 Harvest country music festival after hearing gun fire David Becker/Getty Las Vegas shooting – in pictures People stand on the street outside the Mandalay Bay hotel near the scene of the Route 91 Harvest festival on Las Vegas Boulevard EPA/Paul Buck Las Vegas shooting – in pictures FBI agents confer in front of the Tropicana hotel-casino after a mass shooting during a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures Las Vegas police run by a banner on the fence at the Route 91 Harvest country music festival grounds after shots were fired David Becker/Getty Las Vegas shooting – in pictures An injured person is tended to in the intersection of Tropicana Ave. and Las Vegas Boulevard after a mass shooting at a country music festival Ethan Miller/Getty Las Vegas shooting – in pictures Metro Police officers pass by the front of the Tropicana hotel-casino after a mass shooting at a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures A woman sits on a curb at the scene of a shooting outside of a music festival along the Las Vegas Strip AP/John Locher Las Vegas shooting – in pictures A cowboy hat lays in the street after shots were fired near a country music festival in Las Vegas Getty Las Vegas shooting – in pictures Las Vegas Metro Police and medical workers stage in the intersection of Tropicana Avenue and Las Vegas Boulevard South after a mass shooting at a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures Sheriff Joe Lombardo (2-R) speaking during a press briefing in the aftermath of the active shooter incident on Las Vegas Boulevard EPA

It takes advantage of the force generated by a gun’s recoil - when it kicks backward after firing - and transfers it back to the shooter’s trigger finger, enabling rapid-fire shooting as the stock slides back and forth and causes the shooter to quickly release and then squeeze the trigger.

Bump stocks are not the first gun-modifying devices to draw the attention of gun control advocates.