I think the receiver has to be designated as a pistol grip only, or "other" during the manufacturing process for one thing, and most manufacturers don't want to mess with the ATFE just to prove a point, especially when you're talking about producing something with such limited marketability.
I am not a legal expert. I don't even play one on TV. This is just my own interpretation of the law and your mileage may vary.
You can't legally convert a regular shotgun to less than the required overall length, and you can't use a barrel less than 18 inches. You can't convert a regular shotgun into an AOW (any other weapon) or PGO (pistol grip only) just by adding a 14 inch barrel and pistol grip, or go back and forth between the two. Such designation must come from the factory, but if it did come from the factory designated as an AOW and it is kept that way, then it is legal except where prohibited by local laws.
The moment you change the stock or barrel length you may be changing the legal designation of the firearm. For example, a short barreled, pistol gripped, shotgunish sort of firearm might be designated as an AOW by the factory, but as soon as you mount a regular shoulder stock on that AOW receiver then you might, by definition, be converting your legal AOW into a regulated "Destructive device" and you're not allowed to do that without the right licenses and tax stamps. Anyway, that's my take on the issue, but I'm just another idiot on the internet.
Regards, Alan K.
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