You have the wrong focus/target.
The goal was to create momentum for another "assault weapons" ban, classifying .50USMG rifles as Title II, prohibiting private transfers, expanding the ongoing registration projects, inter alia.
ATFE/FBI/U.S. Attorneys could give a flip about the cartels. They know that they have no jurisdiction and that the cartels cannot be controlled. However, government can control the American public by getting the guns.
Remember this maxim of law enforcement: do what you can, not what is ultimately effective.