The United States Court of Appeals for the 10th Circuit on January 22 heard oral argument in Aposhian v. Barr, a case claiming that the U.S. Department of Justice (DOJ) and the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) violated Article I of the U.S. Constitution when they issued a 2018 rule redefining bump stocks as “machineguns.” By issuing a rule that rewrote federal statute, the plaintiffs argued, the department created a new criminal prohibition beyond the scope of its delegated authority.

DOJ and ATF issued the rule in response to a February 2018 presidential memorandum—an official document that allows the president to manage the federal government—signed by President Donald Trump (R). The memorandum directed the U.S. attorney general to propose a rule banning all devices that turn legal weapons into machineguns.

The New Civil Liberties Alliance (NCLA), a pro bono law firm with a focus on the administrative state, argued that the department lacked the authority to issue the rule because Congress had not delegated power to the attorney general to interpret the scope of the criminal prohibition on machinegun possession. NCLA further contended that the law defining machineguns is unambiguous and, therefore, not open to a new agency interpretation. Since Article I of the U.S. Constitution grants all legislative powers to Congress, argued NCLA, Congress itself must act to change the definition of machineguns to include bump stocks.

Delegate, in this context, means to entrust or hand over authority to another branch of government. Congress sometimes delegates questions requiring subject-matter expertise to agency administrators in order to implement the law. While some scholars support delegation, others argue that legislative authority is vested in Congress alone and cannot be delegated to other branches—a principle known as the nondelegation doctrine.

To learn more about delegation or the NCLA, see here:
Nondelegation doctrine
New Civil Liberties Alliance

Additional reading:
U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives
United States Court of Appeals for the Tenth Circuit
Federal Register 
Final rule
Kisor v. Wilkie