I just installed my 13" VTAC Alpha on my S&W M&P 15T (1st gen) upper. Sliding over the barrel nut was so tight I had to take a 8 oz hammer to the end of the rail to get it on. I have no idea how I'll ever get it off but that's another issue altogether.
The rail is locked down tight and flush with the receiver and I have zero movement at the receiver end of the rail. However, at the muzzle the rail will flex slightly if you push on it. My question is given the flexing at the muzzle end, where I'll have my Troy folding front sight, will the BUIS maintain zero?