The UGS stock rail provides indexing and length adjustment that the rail on a buffer tube would normally provide. By swapping between the 2.00" rail and the Milspec rail on your UGS you are able to reconfigure the stock for the use of Milspec CO2 inserts and Milspec stocks, or 2.00" diameter tanks and 2.00" stocks such as the G&P R3 and similar stock used in paintball applications.
All UGS for M4 models use the original rails.All UGS for HK417 models use the R01 rails.
