NFA rules apply to putting a shoulder stock on a pistol. If you install a 16" barrelled slide assembly along with a stock it is no longer a pistol.
From MechTechs site.
Q) Do I need an FFL (Federal Firearms License) to purchase this product? A) No. The CCU is not a firearm and will not fire without a pistol lower in place. The CCU is simply an accessory such as a scope or grips. For State and local laws you are advised to become aware of them relative to where you live. As far as Mech-Tech is aware, the CCU in and of itself is legal in all States. We do know that in the case of California the combination of the CCU and a pistol lower becomes illegal under current law. If and when Mech-Tech becomes aware of any other special circumstances such as California, we will publish this information. Again, the customer has the responsibility to determine the legality of the combination in their circumstance.
With the above quote taken into consideration and with the length of time MechTech has been selling these units, why are the MechTech units still available to the general public with zero interference from the BATFE?