After the Route 91 massacre, the ATF reclassified bump stocks as machine gun parts. This was done, at least in part, to try and stave off legislation that would have banned not just bump stocks but a whole lot of other things--including, arguably, aftermarket triggers.

But regardless of why it happened, it wasn't the right decision. The bump stock doesn't change a semi-automatic into a full-auto weapon; not based on the NFA definition of a machinegun which is a weapon that is capable or can be easily made capable of firing more than one round with a single pull of the trigger. Bump stocks just let you pull the trigger faster, which is perfectly legal, even now.

Over at The Atlantic, they worry that this case may end up being about more than just bump stocks.

Not so long ago, a case like Cargill would not have come down to whether a court agreed with an agency’s interpretation of a statute Congress had tasked it with enforcing. Indeed, decades of administrative law, including but not limited to the Supreme Court’s 1984 ruling in Chevron v. Natural Resources Defense Council, recognized that agency experts were often in a better position to resolve ambiguities in the statutes that Congress tasked them with enforcing than federal judges were. Thus, it had long been settled that, so long as an agency’s interpretation of ambiguous language in a statute (like what counts as a machine gun) was reasonable, the agency was allowed to act based upon that interpretation.

...

That was already worrying enough, but what’s alarming in Cargill is that the Court is in the midst of getting rid of deference to agencies outside of the “major questions” context, too. Thus, instead of debating whether ATF’s reaction to the Las Vegas shooting was reasonable (which it clearly was), the oral argument before the Supreme Court devolved into the justices struggling to understand the exact mechanical function of a bump stock—so that they could decide for themselves whether or not it fits within the statutory definition of a “machine gun.” As even a cursory perusal of the transcript reveals, this wasn’t a high-minded debate about broader points of law; it was nine neophytes trying to understand the mechanics of something they’ve never touched solely by having it described to them. One comes away from the transcript with the sense that the argument would have been far more productive had it been held on a shooting range. So instead of debating whether the executive branch overreacted or not, the debate was about what, in the abstract, the justices would have done in its place.

Of course, the author clearly frames this as a bad thing. Apparently, if unelected bureaucrats can't essentially determine law by decree, then something is inherently wrong with our country.

Yet this kind of "thinking" is a major problem with our nation in the first place.

Yes, I get the concept that experts may well be better at understanding complex issues than elected officials who, frankly, write a lot of laws about things they don't understand and do a poor job of it.

That's entirely valid.

The problem is that unelected bureaucrats can unilaterally decide something is illegal just so long as they can come up with some reasoning that sort of looks valid. They do this with all sorts of things, not just firearms, but Cargill is a little different. This is something that was declared perfectly legal to sell, then reclassified as illegal simply because it became politically expedient to do so.

If that doesn't highlight the issues with the current system perfectly, I don't know what can.

If the ATF can suddenly decide that this device was legal but now isn't, what's the next thing they'll decide is illegal? 

While ignorant politicians are a danger, our system was created with the idea that they'd be the ones coming up with the laws and not bureaucrats. If this case turns out to be about more than bump stocks, then you're going to have a hard time convincing me that it's a bad thing.