The part where it breaks down is pointing at a specific piece that enables automatic fire and calling that piece a "machine gun", even if it's just a tiny piece of metal or a specially-tied shoelace.
It turns a 'plain gun' into a machine gun, and there are almost no other ways to do that. So it seems like calling it "a machine gun" is reasonable from linguistic perspective. #wittgenstein
The seers have been banned. I don't think anyone thinks they're not machine gun parts.
What's been going on is the ATF has been going after a variety of methods of circumventing the concept--means of using the recoil to "pull" the trigger without the operator actually pulling it. The result sure acts like a machine gun, albeit an unreliable and inaccurate one. The problem is that it's simply too easy to do, they are fighting a hopeless battle.
My understanding is the same problem applies to silencers--plenty of filters out there that just happen to be of the right size to function as silencers. And there isn't even any reason for the rules against silencers. They aren't like Hollywood, it's still loud but below the threshold of hearing damage.
The case entirely hinged on the definition of machinegun being a gun that fires more than one shot "by a single function of the trigger" with the majority saying that because you need to keep forward pressure to keep firing, bump stocks don't count.
The aspect of bump stocks being just a part and not a gun wasn't important to the case because the statute is clear on this point: any part that's intended to convert a regular gun to a machine gun is itself a machine gun under the relevant law.
I think that's already true, except that you probably need standing - you need to show you're affected by the rule - to sue. There are many rules around rule-making including against arbitrary rules, guaranteed public comment periods, etc.
The ATF is simply going on function rather than form. It shoots like a machine gun it is a machine gun no matter what you call it.
That being said, bump stocks are a simple enough concept that banning them is stupid. We should quite our obsession over machine guns--there are few situations where it even matters.
The problem with it going through congress is that it will always be political rather than scientific. The agencies don't do a good job, but a lot of that is because of garbage they are saddled with by congress (think of the machine guns--the basic problem is that the legal and practical definitions are out of sync) and a lot of it is because politics manages to get in anyway.
How about a middle ground: agencies can make rules but they must give their reasoning and supporting evidence--and anyone can challenge such in court. You can't go after the ruling but if you can knock out it's supports it goes away. This would cut both ways--exempt something from a more general ruling and the reason for the exemption can be challenged. (And I'd like to see the same thing for laws.)