According to this article,
http://www.thetruthaboutguns.com/2014/12/jeremy-s/mechtech-c-c-u/"Legality
In this case it is legal to convert your pistol into a rifle configuration and then back into a pistol configuration. The BATFE ruled:
Held further, a firearm, as defined by 26 U.S.C. 5845(a)(3) and (a)(4), is not made
when a pistol is attached to a part or parts designed to convert the pistol into a rifle with a
barrel of 16 inches or more in length, and the parts are later unassembled in a configuration
not regulated under the NFA (e.g., as a pistol).
While it’s specifically illegal to turn a rifle into a pistol, in the case of the CCU the “firearm” part of the system is a pistol and was manufactured as a pistol. Connecting it to the CCU isn’t altering the design of the pistol itself at all, it’s only temporarily switching to rifle configuration via an attachment. No permanent change or redesign is made. ATF ruling is here and further explanation is on MechTech’s forum here."
http://www.mechtechsys.com/forum/viewtopic.php?f=3&t=431