Mass murderer Stephen Paddock modified at least one of the 23 guns stashed in his Las Vegas sniper’s nest, adding legal gadgets that allowed him to fire faster and squeeze off more bullets at a time.

One of the guns that the retired accountant used to kill nearly 60 people at a county music festival was an AR-15 rifle, which he had outfitted with a “bump stock,” making it capable of rapid fire.

“We are aware of a device called a bump stock . . . that enables an individual to speed up the discharge of ammunition,” Clark County Sheriff Joe Lombardo said at a press conference Tuesday.

Paddock, who had bought several guns legally over the last year, also attached a special clip to the rifle capable of holding 50 to 75 rounds of ammunition — and a high-tech electronic scope.

“It’s been tricked out a little bit,” explained Dave Workman, senior editor of GunMag.com.

A photo of Paddock’s AR-15 rifle shows that it included a forward front grip, which is “typically” added by gun buyers, as well.

“All that does is allow a shooter to get a better hold on the front end during recoil,” Workman said.

“It helps in maintaining a sight picture, which comes in handy during competition or defensive situations. Lots of cop guns have those, too.”

Law enforcement found 23 guns in Paddock’s 32nd-floor room at the Mandalay Bay Resort and Casino, which he had hauled up in 10 suitcases when he checked in Thursday without arousing suspicion.

The high-powered weapons also included a .308-caliber AR-10 rifle and an AK-47-type rifle — as well as four Daniel Defense DDM4 rifles, three FN-15s and other rifles made by Sig Sauer, according to reports.

Police found two bump stocks, which are sold and manufactured legally, inside his room, according to the Associated Press.

The devices alter the gun’s stock so the recoil helps accelerate how quickly the shooter can pull the trigger.

“All the guns he had were very expensive, from $5,000 to $10,000 each, which is a lot of money for a gun,” a law enforcement source told The Post. “A lot of them were from a company called [Lewis Machine & Tool Co.].’’

All of the rifles were “high end” and typically used by people in law enforcement professions, the source added.

In chilling footage obtained by The Post, an assault rifle resting on a bipod can be seen on the floor through the breached door of Paddock’s hotel room, which is criss-crossed by yellow crime-scene tape.

A marker with the number 19 is seen resting near the weapon — one of the many found along with the body of the madman, who killed himself with a revolver before SWAT team members stormed his room.

Las Vegas Sheriff Joe Lombardo couldn’t say whether any of the weapons had been converted into fully automatic rifles — but added that ATF agents were analyzing Paddock’s guns on the East Coast.

Newly manufactured fully automatic weapons have been banned for civilian use since the Firearm Owners’ Protection Act of 1986.

Ed Turner, a former police officer who owns a gun shop in Stockbridge, Ga., told the AP he’s seen an increase in demand for bump stocks since the Sunday evening massacre. He guessed he had sold two of them over the last decade — but now even wholesalers are out of them.

Sen. Dianne Feinstein (D-Calif.) spoke out against bump stocks Tuesday, calling for a ban on their sale.

“Individuals are able to purchase bump fire stocks for less than $200 and easily convert a semi-automatic weapon into a firearm that can shoot between 400 and 800 rounds per minute and inflict absolute carnage,” she said.