From my understanding, once you attach a stock to a receiver, it always has to be classified as a long gun. Remington, Mossberg, and many other manufactures are using a loop hole. If you never attach a stock to it, you can use a stabilizing brace or other funky style pistol grip and then the gun is classified as a firearm, and not a pistol or a shotgun or rifle. So they use new receivers to manufacture these "firearms". They also have to be a certain length. Going off memory, the gun must be at least 26 inches or so long. If its at least that long, then no govt paperwork is required. Its not a short barrel shotgun. Not a pistol. Classified as a firearm.
Are those still classed as just a "firearm" even with the pistol grip?
That's how I understand it. Remington got it approved for sale through the ATF, so its good to go. The stabilizing brace is not considered a stock per the law, so its not a short barrel shotgun.