Short version: ATFE sez it’s an arm brace..until you put it up to your shoulder…then you have magically ‘redesigned’ it into a shoulder stock and you’re treading into NFA country.
So..according to the weasels at ATFE…how you use something can ‘redesign’ it. It’s like ATFE has singlehandedly turned every household item into some sort of Transformer. Car out of gas? You’ve redesigned it into a sculpture. Taking a wiz on a campfire? You’ve redesigned your johnson into a fire extinguisher.
This will be interesting to see play out.