And now, in the March/April 2015 issue of
American Handgunner magazine, an article by Will Dabbs, MD covering the Sig P516 close combat weapons system, states that according to a recent ruling by the BATF's Firearms Technology Branch, SIG's Pistol Stabilizing Brace does
not classify the host gun as a short-barreled rifle " . . . even if used as a short buttstock in tactical applications . . . "
He mentions that a copy of the BATF's ruling accompanies the device.
The author also observes that the buttstock bears (in his words) " . . . a striking resemblance to a collapsed M4 stock . . . "
So . . . we have at least two gun rags weighing in on the issue now, so I hope everything is perfectly clear.
(Of course, a couple of years ago a gun rag printed a picture of an AR-type pistol with a forward grip attached, making it an AOW . . . so we know how we can rely on them for legal advice.)