"Kind of like closing the barn door after the horse is gone," said firearms trainer Don Pind.
Bump stocks work by taking the back part of the rifle, or stock, off and replacing it with the device.
The action of the gun sliding back and forth in the stock from the recoil causes the shooter to pull the trigger more quickly.
A 2014 YouTube video shows the bump stock in action.
In the video, the shooter demonstrating the bump stock says, "It did take a little bit of adjustment in the loading of the mechanism to get it to slide smoothly, but once we got it was working pretty well for us."
Investigators say the Las Vegas shooter used rifles with bump stocks to fire more bullets more quickly.
Pind told 41 Action News the shooter would've had to practice with the device to fire as many rounds as he did.
He also says Show Me Shooters Indoor Range in Claycomo, Missouri stopped selling them about two years ago.
"Everyone that we sold them to probably half of them brought them back, so we quit handling them," Pind said.
The 41 Action News Investigators spoke to more than a dozen metro firearms stores and found only one had a few bump stocks.
The owner, who didn't wish to be identified, said the devices sold out quickly after public discussion of banning them.