The DS is recommended when not running anything below a 13.3 barrel. Plus, its rated for full-auto shooting. The main differences from the DSX are the absence of the secondary tube retention ring and the use of Grade 9 titanium on the tube instead of Grade 5. Grade 5 titanium is much more expensive, and it simply isn't needed in the DS series.
Caliber : 5.56 NATO
Diameter : 1.75"