So BATF says that if the "shotgun" came from the factory without a shoulder stock of any kind, and only a pistol grip, it's technically not a shotgun at all. Shotguns have to be designed and intended to fire from the shoulder.
If it's not a "shotgun" then it can't be a "sporting shotgun" and get the "sporting shotgun" exception to the rule about how guns with a bore diameter over .50 caliber are "destructive devices."
Conclusion: There's a danger that BATFE will declare your pistol-grip-only shotgun a "destructive device" like it were a cannon, bazooka, anti-tank rifle, etc. And without an amnesty period, they could just start prosecuting people right away. No warning, no notice, just start enforcing "the law" as they have reinterpreted it.
MY TAKE: I suppose BATFE could try this, and with the leftist mass media on their side they could probably avoid the polictial fallout.
But if we are worried about BATFE going evil on us, I have to point out that there are bigger dangers that would affect more guns.
For example, the current method of measuring barrel length includes the chamber. It goes from the muzzle to the breech face of the closed bolt, closed on an empty chamber.
What if BATFE decided that "barrel" only included the part of the bore that the projectile touches on its way out?
Then the chamber would not count. Then your muzzle accessory on the end may not count. What had previously been a 16.1" carbine barrel might now be factually determined to be a 14" barrel. Shotguns have chambers 2.5" long. Your 18.5" shotgun barrel would only be 16" if you didn't count the chamber.
And all BATFE has to do is re-interpret the law and say that they've come to a different and more accurate conclusion than they came to in the past.
P.S. I'd like to hear JRM's take on all of this. He represented a company who was making a product that BATFE specifically approved of one year, and then later changed their mind and said it was an illegal product. So John would know what standards apply when BATF changes its mind and wants to make something illegal that it has always said was legal before.