Clark Aposhian added a bump stock to his firearms collection about a year before a gunman using the devices, which let semi-automatic weapons fire more rapidly, killed nearly five dozen people at a Las Vegas music festival.

Clark Aposhian. (AP Photo/Rick Bowmer)

Today, the 54-year-old gun lobbyist is the only person in the U.S. legally allowed to own one. The exception comes via an order from the 10th U.S. Circuit Court of Appeals that granted a temporary exemption to a Justice Department ban following the massacre. The ruling applies exclusively to him and is for the “time required to adequately consider and rule on” a pending motion.

Other gun rights advocates and organizations have attempted to obtain a similar exemption to no avail, though their legal challenges and Aposhian’s are continuing in federal courts from coast to coast. The Supreme Court alone received three different requests to halt enforcement of the bump stock ban and rejected all of them.

Rather than arguing that bump stocks are protected by the Second Amendment's guarantee of the right to gun ownership, Aposhian and his lawyers say the ban is unconstitutional because of how it was imposed. President Trump asked the Justice Department to prohibit the devices in February 2018, four months after the Las Vegas deaths and in the aftermath of the massacre of 17 people at a south Florida high school.

The department gave Trump what he wanted that December. It modified the definition in Bureau of Alcohol, Tobacco, Firearms and Explosives rules of “machine guns" outlawed by Congress so that it covered bump stocks, giving owners three months to destroy or turn in to federal authorities some 280,000 to 500,000 of the devices in circulation.

[Related: Supreme Court rebuffs request from gun rights advocates on bump stock ban]

The rule change, under then-acting Attorney General Matthew Whitaker, marked a departure from the Justice Department’s previous position that bump stocks could only be banned by Congress. It's a shift Aposhian is taking advantage of.

“Bump stocks are for the most part a placeholder in this lawsuit,” he said. “What this is about is government overreach. This is about the constitutional or the statutory authority that was lacking for the executive to promulgate such a rule.”

Mark Chenoweth, executive director of the New Civil Liberties Alliance, which is representing Aposhian, fears that if the courts side with the Justice Department, it may open the door for more agencies to effectively rewrite laws passed by Congress.

“If the courts allow the executive branch to color outside the lines in this way, you can be guaranteed they'll do it in the future, regardless of the context,” he said.

Clark Aposhian attaches a "bump stock" to a semi-automatic rifle. (AP Photo/Rick Bowmer)

Aposhian, who bought his Slide Fire bump stock on clearance for less than $200, said the device came with a letter from the firearms bureau, commonly referred to as ATFE, informing him it could be legally owned and operated. The bump stock initially sat in his gun room, but since the Las Vegas killings, he has used it a handful of times, primarily for demonstrations to reporters seeking to understand how the accessory works.

“This is Congress’s job to do,” he said of the ban. “If you’re going to change the law, it is absolutely and specifically the purview of Congress and our representatives, not the role of the president, not via executive order, let alone by an executive agency, to do this.”

[Also read: House approves domestic violence law with new gun restrictions]

If Aposhian loses his court battle, he will surrender his bump stock, Chenoweth said. But he fears for people who still have the devices and aren’t aware of the new rule.

"What about all those people out there — we know there are 500,000 — who purchased this device, who got a letter from the ATF saying this is legal, and they're not following current events like you and I do?” Chenoweth said. “One of those people is going to show up at a gun range if this law is upheld, minding their own business, using their own bump stock, not realizing they're a felon now, and they're going to get arrested and face a 10-year prison sentence."