Austin gun rights activist Michael Cargill on Monday walked into a federal building in Northwest Austin and surrendered two bump stocks on the eve of federal authorities' deadline to divest of the weapon attachments.

But Cargill said he hopes to get them back.

Cargill is suing the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives in response to the Trump administration's ban on "all devices that turn legal weapons into machine guns,” arguing that the question of whether to ban bump stocks should be up to Congress. The rule went into effect last year.

The bump stock — the attachment used by the killer in the 2017 Las Vegas massacre to make his weapons fire rapidly like machine guns — becomes illegal Tuesday. It's the only major gun restriction imposed by the federal government in the past few years, a period that has seen mass shootings in places such as Las Vegas; Thousand Oaks, Calif.; Sutherland Springs; Orlando, Fla.; and Parkland, Fla.

Unlike the decadelong assault weapons ban, the government isn't allowing current owners to keep their bump stocks. They must be destroyed or turned over to authorities, and the government isn't offering any compensation for the devices, which can cost hundreds of dollars. Violators can face up to 10 years in prison and thousands of dollars in fines.

Cargill, who runs the South Austin firearms store and academy Central Texas Gun Works, said he purchased the two bump stocks last year, soon after learning about the ban.

Cargill said his lawsuit is "really not about the bump stocks."

"It's about how the federal government has said they're banned, and they're not going to reimburse you for it," he said. "An enforcement agency — which is what the ATF is — is creating laws, and this must stop. We have a Constitution here. If we allow an agency within our government to create laws, that means they can do whatever they want, and the people will have no recourse."

Cargill said his lawsuit would be filed by the end of Monday. He is being represented by the nonprofit New Civil Liberties Alliance.

ATF spokeswoman Nicole Strong declined to say how many bump stocks have been surrendered in the Central Texas area. The American-Statesman has requested the information under the Freedom of Information Act.

"The number turned in doesn't take into account the number that people are destroying on their own or turning in to local law enforcement," Strong said.

The ATF recommends that bump stock owners destroy the devices by following instructions for how to do so at www.atf.gov. Cargill said he recommends that bump stock owners turn them over at ATF offices, which Strong said is also an option. Anyone wishing to turn in a bump stock to the local Austin ATF office — at 9009 Mountain Ridge Drive, Suite 220 — should call 512-231-2880 and make an appointment.

Strong declined to comment on Cargill's pending litigation.

Bump stocks were originally created to make it easier for people with disabilities to fire a gun. The device essentially replaces the gun's stock and pistol grip and causes the weapon to buck back and forth, repeatedly "bumping" the trigger against the shooter's finger.

Technically, that means the finger is pulling the trigger with each round fired, a distinction that led the ATF to allow the devices.

They were considered by most gun owners to be a novelty and weren't widely known until the Las Vegas gunman attached bump stocks to several of the rifles he used to rain bullets on concertgoers outside his high-rise hotel room.

Additional material from The Associated Press.