Sig Sauer just shared a letter sent to a police department from the ATF’s Firearms Technology Branch concerning the use of arm braces that referenced Sig’s own SB15 pistol stabilizing brace for AR pistols specifically.
The ATF’s letter says that they’ve determined that firing a pistol from the shoulder would not cause the pistol to be reclassified as an SBR. The letter, signed by the Chief of the ATF’s Firearms Technology Branch, Earl Griffith, states, “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.”Gear Scout has a copy of the letter. Key point here, "...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.” How you use the AR-style pistol does not (and by my reading cannot) change the classification of the firearm as a pistol and not an SBR.
The reason I bring this up is that on a private email list this very question came up recently, to wit, if you shoulder the buffer tube of a pistol does it change the classification of the firearm. In fact, an AR manufacturer had stated that they had been informed (verbally) by ATF that using a pistol off the shoulder changed the classification. This letter clearly negates that claim. At least for right now, the the letter states that the physical characteristics define whether a firearm is a rifle, pistol or short-barreled rifle, not how the firearm is used:
FTB classifies weapons based on their physical design characteristics. While the usage/functionality of the weapon does influence the intended design, it is not the sole criteria for determining the classification of a weapon. Generally speaking, we do not classify weapons based on how an individual uses a weapon.