I'd wager you're not going to find an ATF agent that will arrest/charge someone solely with, "your brace makes your pistol a short barreled rifle". Yeah, if you're caught trafficking drugs, whores, machineguns, etc., they'll tack it on with the rest, knowing it will disappear in a plea agreement.We’ll lets figure out what jurisdiction to get you arrested in and get things started.
This is a dangerous game of poker that the ATF is playing because, at this point, a first year law student at the bottom of his class could make a very solid argument that the entire NFA is 'void for vagueness'.
The ATF will go back to busting moonshiners if the NFA is thrown out. The only thing left of the agency will be a handful of diverse women and a fax machine.