here is an article From S. Wenger and DUF mailer on the subject along with a link to D. Corderas blog
Own a Pistol-Grip Shotgun?: An interpretation by the Bureau of Alcohol, Tobacco, Firearms and Explosives that pistol grip shotguns are not shotguns has created an unforeseen legal liability for owners of such firearms. ATF’s Nov. 2009 FFL Newsletter declared: “Certain commercially produced firearms do not fall within the definition of shotgun under the GCA even though they utilize a shotgun shell for ammunition. For example, firearms that come equipped with a pistol grip in place of the buttstock are not shotguns as defined by the GCA.” Here’s another wrinkle, from Mike Vanderboegh at Sipsey Street Irregulars: “An October 27, 2010, letter from the Firearms Technology Branch ruled that such a firearm, with a 17" barrel and 26-1/4" overall length, was not subject to the National Firearms Act.” …That would seem to indicate there’s no issue with violating National Firearms Registration and Transfer Record (NFRTR) requirements, right? No worries if you own one, or want to buy one…? Not so fast. If the pistol grip firearms are not “shotguns,” what are they? … (Personally, I’ve only seen one person – a list member I count among my mentors – who could hit reliably at distances better measured in yards than in feet with a 12-gauge shotgun that lacked a buttstock.)
http://www.examiner.com/gun-rights-in-national/atf-position-on-pistol-grip-shotguns-creates-new-danger---