It’s still early but this seems to be detailed enough to be believed:
Stephen Paddock, the Las Vegas shooter, had an arsenal of 17 weapons in his hotel room, mostly military-style rifles, according to a law enforcement source.
At least one of them had been modified with a legal “bump stock” style device that allows the shooter to rapidly fire off rounds without actually converting it to a fully automatic weapon, the source said.
The devices modify the gun’s stock so that the recoil helps accelerate how quickly the shooter can pull the trigger. The devices are legal in the U.S.
Other weapons may have been converted to fully automatic fire, and were still being examined, the source said.