There isn't one
Here is the atf according to tfb
"After examining a sample Q Honey Badger Pistol, the ATF’s Firearms and Ammunition Technology Division (FATD) concluded that “the objective features of the Honey Badger firearm, configured with the subject stabilizing brace, indicate the firearm is designed and intended to be fired from the shoulder.” FATD further concluded that it is a SBR as defined by the National Firearms Act (NFA) and Gun Control Act (GCA). Yet, ATF does not explain how it arrived at this conclusion other than vague generalizations that the firearm was “designed” to be fired from the shoulder and by virtue of its barrel length meeting the definition of a SBR. "
They literally have themselves this power under order of trump. There is nothing about a bumpstock that makes it a machine gun, but here we are.
Let me preface my response to the part I highlighted by saying that I fully support the total repeal of the NFA and GCA. Repeal without replacement, and nullification of all state level gun control through federal preemption.
However, I never believed for a second that pistol braces were designed to be anything other than a shoulder stock that skirts the NFA. I don't have a problem with that. I didn't have a problem with AWB ban-era neutered ARs either. The government makes a loophole, I'm all in favor of driving a freight train through it.
But let's be honest. Braces were designed to be shouldered, while thumbing our collective noses at the letter of the law. If you claim otherwise, you're being disingenuous.