What is a Mil-Spec Buffer Tube?
A mil-spec tube is just as the name implies, made to the specifications
of the technical data package (tdp). The biggest concern is the diameter
of the tube, the milspec part has a diameter of ~1.147, the thread
diameter is 1.1875

What is a Commercial Buffer Tube?
During the original cloning of the military AR, the first commercial receiver extension tubes were
made from an extrusion, the threads lathe cut and the body was not
reduced. The common size for this is 1.170 and that has became the de
facto standard for after market tubes.