The effectiveness of the external shroud depends on preserving useful volume within the surrounding shroud. So, using a 16mm LW barrel wouldn't be practical, as it would take too much space. Obviously, then, the GD LW barrels are of lesser diameter. Not as thin as the factory Crosman barrel, but thinner nonetheless. But thick enough to avoid detrimental barrel harmonics.
I know the actual thickness... I just don't see why it's important to know, as long as it works well.