Today, at the NFATCA Firearm Importer, Exporter and Manufacturer Conference, the BATFE announced that Acting Director Melson issued Ruling 2011-4, regarding whether a pistol can be converted to a rifle and then back to a pistol, without having to register the firearm as a short-barreled rifle. While the Ruling has not yet been published,ATF Deputy Chief Steve Albro stated that the Ruling holds that a pistol may be converted to a Title I rifle (non-SBR) and back to a pistol without having to register the firearm as a Short-Barreled Rifle under the NFA. However, if the firearm starts life as a rifle, the Ruling further holds, that it cannot be converted to a pistol without registering it as a Short-Barreled Rifle under the NFA. As soon as the Ruling is posted, I will update this posting with a copy and any additional holdings.
UPDATE: The BATFE has published 2011-4 here