Um.... it
is up to congress.
The ATF director did not budge, responding, "Respectfully, that is a decision that different legislative bodies have come up with different definitions for. It would be for the legislators to make that determined action as to how they would define it unless they were to delegate that authority to ATF." . . . . "I’ll go shorter than that, because honestly, if Congress wishes to take that up, I think Congress would have to do the work, but we would be there to provide technical assistance. I, unlike you, am not a firearms expert to the same extent as you maybe, but we have people at ATF who can talk about velocity of firearms, what damage different kinds of firearms cause, so that whatever determination you chose to make would be an informed one." Dettelbach answered.
This is true.
@Roadvirus, it is a literal statement of how lawmaking works. Either congress defines it and they delegate it to the relevant agency's rulemaking authority. The prior assault weapons ban was spelled out in statute.