Saw this posted over at calguns, thought I'd propagate the knowledge here:
"open_letter_on_the_redesign_of_stabilizing_braces "
http://www.atf.gov/sites/default/fil...ing_braces.pdf
The core info:
The pistol stabilizing brace was neither “designed” nor approved to be used as a shoulder stock,
and therefore use as a shoulder stock constitutes a “redesign” of the device because a possessor
has changed the very function of the item. Any individual letters stating otherwise are contrary
to the plain language of the NFA, misapply Federal law, and are hereby revoked.
-- Michael