According to Firearmchronicle
Now, according to a new interpretive rule from the agency, owning a bump stock is forbidden by a longstanding federal statute that outlaws the “possession [of] a machinegun.” Whether bump stocks can be fairly reclassified and effectively outlawed as machineguns under existing statutory definitions, I do not know and could not say without briefing and argument. Nor do I question that Congress might seek to enact new legislation directly regulating the use and possession of bump stocks.
But at least one thing should be clear: Contrary to the court of appeals’s decision in this case, Chevron U. S. A. Inc. v. Natural Resources Defense Council, Inc. 467 U. S. 837 (1984), has nothing to say about the proper interpretation of the law before us. In the first place, the government expressly waived reliance on Chevron.
The government told the court of appeals that, if the validity of its rule (re)interpreting the machinegun statute “turns on the applicability of Chevron, it would prefer that the [r]ule be set aside rather than upheld.”