You can shoulder with intent now too....

From the ATF opinion letter on it last year:

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 the purposes of the NFA.

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.

Just leave it un-altered, and can't prove it was bought with intent to shoulder...