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.
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.