In March 2014, the ATF determined correctly that a stabilizer brace – was not a shoulder stock, in 2015 the ATF issued a vague “clarification” that shouldering a stabilizer “made” it into a stock. Fast forward to March 2017 and the ATF has now determined that shouldering a brace is legal, since the act of shouldering something does not redesign it.
So yes, its 2017, and you now can shoulder a pistol brace. What you cannot do according to the ATF is; “take affirmative steps to configure the device for us as a should-stock”. Some examples provided by this inconstant federal agency would be removing the arm-strap or permanently fixing the brace so that it cannot be used as a stabilizer.
The bottom of Page 2 is where it gets specific:
“With respect to stabilizing braces, ATF has concluded that attaching the brace to a handgun as a forearm brace does not “make” a short-barreled rifle because in the configuration as submitted to and approved by FATD, it is not intended to be and cannot comfortably be fired from the shoulder. If, however, the shooter/possessor takes affirmative steps to configure the device for use as a shoulder-stock for example, configuring the brace so as to permanently affix it to the end of a buffer tube, (thereby creating a length that has no other purpose than to facilitate its use as a stock), removing the arm-strap, or otherwise undermining its ability to be used as a brace – and then in fact shoots the firearm from the shoulder using the accessory as a shoulder stock, that person has objectively “redesigned” the firearm for purposes of the NFA. This conclusion is not based upon the mere fact that the firearm was fired from the shoulder at some point. Therefore, an NFA firearm has not necessarily been made when the device is not re-configured for use as a shoulder stock – even if the attached firearm happens to be fired from the shoulder.”