Here is the BATFE definition of pistol for purposes of the NFA:
"§ 479.11 Meaning of terms.
* * * * *
Pistol. (a) A weapon originally designed, made, and intended to fire a projectile (bullet) from one or more barrels when held in one hand, and having--
(1) A chamber(s) as an integral part(s) of, or permanently aligned with, the bore(s); and
(2) A short fixed stock designed to be gripped by one hand and at an angle to and extending below the line of the bore(s).
(b) The term shall not include any weapon disguised to look like an item other than a firearm, such as a pengun, wallet gun, belt buckle gun, pager gun or gadget device, or any gun that fires more than one shot, without manual reloading, by a single function of the trigger. "
I do not see how they can declare, say an AR type pistol, with a fore grip added later as an aow.