Earlier this year, Sig Sauer started marketing an AR-15 pistol with a "stabilizing brace" that looks a bit like a short stock. As pictured here, it's designed to fit over the forearm.
People are claiming that the ATF, local law enforcement, or Bigfoot have been cruising public ranges and making arrests because some folks are using the brace as a stock. That's not true. They can't do that.
The ATF has classified this weapon as a pistol. Sticking it up to your shoulder does not turn it into a short-barreled rifle. I've already had arguments about this, so here's the response from the ATF themselves:
FTB [Firearms Technology Branch] has previously determined (see FTB #99146) that the firing of a weapon from a particular position, such as placing the receiver extension of an AR-15 type pistol on the user’s shoulder, does not change the classification of a weapon. Further, certain firearm accessories such as the SIG stability brace have not been classified by the FTB as shoulder stocks and, therefore, using the brace improperly does not constitute a design change. Using an accessory improperly would not change the classification of a weapon under Federal law.
The relevant word is designed. As per 26 U.S. Code § 5845(c), a rifle is a weapon that is designed to be fired from the shoulder. This doesn't qualify.