The 12" barrels would worry me for NFA. I'd want to talk to tech branch first.
You know I thought about that... but as someone said to me, its not a rifle. It dont have a stock on it, not a shoulder fired gun. So I would think it would be more on the lines of a "other firearm".
This is what is said on RGG web page, the place that makes them, and sales the blue prints to make your own... "BATF has stated that a gun built to our design is considered a "Title I firearm" which doesn't require a license or registration to build or own."