Let me say up front that we absolutely should challenge this ruling. SAF is already doing so and I support that.
Yes, this will be challenged and we will get a ruling - only then can we tell how far the precedent will go. That said, I respectfully disagree with those who are saying that, if upheld, it will allow ATF to ban anything they want. Let me address some of the concerns I have seen in posted online.
"Any semi-auto can be bump fired, so this allows any semi-auto to be classified as a machine gun!"
Nope. The law and the ruling address devices. A bump stock effectively allows you to pull the trigger once and hold it, resulting in the firearm pulling its own trigger. Semi-auto firearms are well defined in law, and the fact that they can be fired rapidly without the use of a device does not change the definition.
"When the standard is, ‘Its a machine gun if we say its a machine gun’ then what is stopping them from again changing the language for the definition of a ‘machine gun’?"
Answer: The language of federal law. This rule does not simply say, "A machine gun is whatever we say it is". It makes an argument that bump stocks are similar to auto sears, in that they allow the user to pull and hold the trigger to fire multiple rounds. We shall see if the courts agree - but even if upheld, the scope will be limited to devices that enable semi-autos to emulate full auto fire.
"What is stopping them from declaring all semi-automatic firearms to be machine guns?"
Answer: The explicit definition of a semi-auto firearm in federal law. That trumps any ruling by ATF.
"What is stopping them from declaring any gun that has the ability to accept a detachable magazine to be a machine gun?"
Answer: The fact that the definition of a machine gun in federal law says nothing about magazines. Legally, a machine gun may have a fixed magazine.
"What is stopping them from declaring anything that can be fired more rapidly than a muzzle loader to be a machine gun?"
Answer: Really? Let's get real - this ruling is quite literally, rooted in law. An argument can be made that bump stocks are a device that turn a semi-auto firearm into a full auto one. This places them in the same category as auto sears. The fact that an auto sear is a class 3 item does not make any firearm that can accept them a class 3 item. That argument may be found to be invalid by the courts, but even if upheld, it would only extend to devices that effectively convert semi-autos to full autos.
We should challenge this ruling - but do not think that if we win, bump stocks will be legal for long. Congress will quickly pass, and Trump will sign, a law to the same effect, Unless the high court decides to change its ruling in Heller, bump stocks will be found to fall outside the scope of 2nd Amendment protection. Why? Simple, they fail the "common use" test. I'm not saying I agree with this - just that it is what will likely happen.
So, why challenge the ruling? Simple, we might lose on the issue, or we might win - but either way, we are likely to get a ruling that extends 2A protection. For instance, SCOTUS might rule that bump stocks may be banned, but make it clear that all semi-auto long guns are protected. Comments by recently confirmed SCOTUS justices affirm that semi-auto rifles are protected. They could make the same statement even if the rule is found to be unconstitutional.
In any case, this ruling is not as bad as many fear. The sky is not falling. Meanwhile, we have many other, more important battles to fight. Let's not be distracted.