Beg to differ
With the case of thompson/center vs US. The courts ruled that a firearm registered as a pistol can be made into a rifle by the addition of a 16"+ barrel and then have a stock attached. It can then be made back into a pistol by reversing the process. Case law here, not legal conjecture.
With an AR type firearme the process is remove the pistol length upper.. Put on the 16" upper and then slide your stock on. As long as the "short" upper and stock are never on at the same time, all is legal.
Having a carbine buffer tube on a pistol lower is 100% legal and facilatates this process.