Hoping this graphic is helpful in pointing out the differences between the two:


buffer.tube.jpg

There was an extended thread on this some years back. As it happened, our armorers were doing the semi-annual detail strip, cleaning, inspection and re-assembly of our ARs.

Now, on the thread specs and "full depth of thread" on the mil-spec receiver extensions: I measured something over 20 Colt mil-spec receiver extensions/buffer tubes. Exactly one was the diameter listed above, the rest were under size. A number of those had less thread engagement than a commercial version. There's the drawing, then there's the execution.

The commercial versions generally have an appropriate class 2 thread fit IIRC.

There's not much point in beating this dead horse further.