You can't go wrong with a DPMS Oracle, Sportacle or LoPro from Midway $ 360 to 375. Looks like they have stock & they all get good reviews. I started out with a 16" SS lowpro it was on the heavy side - but a great shooter, cycled any and all ammo including steel case. Beware of the DPMS haters!
I have a NFA lower too, I've run it with three of my uppers to see how reliable it would be, ran fine with every one. I think if you end up with the DPMS heavy barrel lopro upper, once you add optics, it will be front heavy when paired with the NFA. The Oracle would ultimatly be my choice for the best bang for your buck.