Classified as AOW, any other weapon. You cannot put a stock on it, however, as that makes it a short barrelled shotgun which falls under the 1934 NFA.
I don't think so. The
Any Other Weapon classification requires a tax stamp, whereas the Shockwave and Tac-14 require no tax stamps. They constitute a true loophole, not fitting any Federal classification category other than "firearm," for which there are literally no specific Federal regulations apart from those applying to all firearms equally, which is a very short list. Basically, felons cannot own one, and they cannot be shipped directly to a non FFL holder, and that's about it.
