Mil-Spec buffer tubes have a diameter of approx. 1.14”, and generally have a flat back and the threads rise above the tube surface.
Commercial-Spec buffer tubes have a diameter of approx. 1.17”, and generally have a slanted back and the threads are level with the tube surface.
For a mil-spec stock you need a mil-spec buffer tube, for a commercial-spec stock you need a commercial-spec buffer tube, some stock manufacturers such as FAB Defense make stocks with friction lock systems that will fit both mil-spec and commercial-spec tubes.
