FTB 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. However, the FTB cannot recommend using a weapon (or weapon accessory) in a manner not intended by the manufacturer.
The Letter that Sig sends with the SB-15 states that it is ok to use the brace as intended and does not create a firearm subject to the NFA. Previously this left many concerned that if someone was to use the brace not as intended an SBR would be created. This letter from the ATF clears up the issue and states that just because an accessory was not used as intended, the firearms is not reclassified by ATF.
If you would like a copy of the letter from the ATF for your records it can be downloaded.