Machine guns?
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.)