From The Truth About Guns:

In his opinion for the majority, Justice Clarence Thomas wrote what most people who understand and are familiar with firearms have been thinking all along.

“Semi-automatic firearms, which require shooters to reengage the trigger for every shot, are not machine guns,” Justice Thomas wrote. “This case asks whether a bump stock—an accessory for a semi-automatic rifle that allows the shooter to rapidly reengage the trigger (and therefore achieve a high rate of fire)—converts the rifle into a ‘machine gun.’ We hold that it does not.