There have been many question on converting a pistol to a rifle and back to a pistol. The ATF recently issued a ruling regarding this to help clarify what is permitted and what is not. If you would like to read the full ruling it can be found here. ATF Ruling 2011-4.pdf
In summary the ATF made the following findings.
Held, a firearm, as defined by the National Firearms Act (NFA), 26 U.S.C. 5845(a)(3), is made when un-assembled parts are placed in close proximity in such a way that they:
(a) Serve no useful purpose other than to make a rifle having a barrel or barrels of less than 16 inches in length (e.g., a receiver, an attachable shoulder stock, and barrel of less than 16 inches in length); or (b) Convert a complete weapon into such an NFA firearm, including – (1) A pistol and attachable shoulder stock; and (2) A rifle with a barrel of 16 inches or more in length, and an attachable barrel of less than 16 inches in length.
Such weapons must be registered and are subject to all requirements of the NFA.
Held further, a firearm, as defined by 26 U.S.C. 5845(a)(3) and (a)(4), is not made when parts in a kit that were originally designed to be configured as both a pistol and a rifle are assembled or re-assembled in a configuration not regulated under the NFA (e.g., as a pistol, or a rifle with a barrel of 16 inches or more in length).
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 un-assembled in a configuration not regulated under the NFA (e.g., as a pistol).
Held further, a firearm, as defined by 26 U.S.C. 5845(a)(4), is made when a handgun or other weapon with an overall length of less than 26 inches, or a barrel or barrels of less than 16 inches in length, is assembled or produced from a weapon originally assembled or produced only as a rifle. Such weapons must be registered and are subject to all requirements of the NFA.