The U.S. Supreme Court grappled with the technical aspects of bump stocks during a hearing Wednesday on whether the federal government can ban them as “machine guns,” with a few conservative justices seeming skeptical that the rapid-fire enabling devices meet that definition.

The U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives banned bump stocks in 2018 after an assailant used one in killing 60 people at an outdoor Las Vegas music concert—still the deadliest mass shooting by a single perpetrator in U.S. history. The devices modify semi-automatic firearms to fire continuously with one pull of the trigger.