The Gun Control Act of 1968 (GCA), 18 U.S.C. § 921 (a)(7), defines the term “pistol” as: A weapon originally designed, made, and intended to fire a projectile (bullet) from one or more barrels when held in one hand, and having (a) a chamber(s) as an integral part(s) of, or permanently aligned with, the bore(s); and (b) a short stock designed to be gripped by one hand and at an angle to and extending below the line of the bore(s).
Additionally, § 921 (a)(7) defines the term “rifle” in part as a weapon designed or redesigned, made or remade, and intended to be fired from the shoulder....
Further, the National Firearms Act (NFA), 26 U.S.C. § 5845(a), defines the term “firearm” to include: ...(3) a rifle having a barrel or barrels of less than 16 inches in length…[and] (4) a weapon made from a rifle if such weapon as modified has an overall length of less than 26 inches or a barrel or barrels of less than 16 inches in length....
Research by FTB indicates that the CCU consists of one of three shoulder-stock variants and a replacement barrel of 16-1/4 inches. (Please consult our enclosure of two pages containing data and images obtained from the Mech Tech Systems, Inc., web site.) The conversion unit is designed to work with either a 1911-type pistol or a Glock-patterned pistol, converting either into a “rifle” configuration. The conversion kit itself is simply a collection of firearm parts not regulated under the GCA or NFA.
As you can see from the citations noted above, there is no definition of a “weapon made from a pistol.” Thus, converting a pistol into a rifle using the CCU would not be a violation of the GCA or NFA. However, there is a definition of a “weapon made from a rifle.” Therefore, when you convert a pistol into a rifle, it can not then be converted back. Doing so would constitute the making of an NFA-class firearm that would require prior approval by ATF and appropriate registration.