This product is not compatible with the Protac Rail Mount HL-X. A separate version for the HL-X is available here. This product is not compatible with the Protac 2.0 Rail Mount Lights, but IS compatible with the Protac Rail Mount 2 Lights. Please double check the name of your light before ordering. This adapter requires the use of a Surefire UE or DS tailcap to function, which is not included. The Arisaka Tailcap Adapter allows Streamlight Protac Rail Mount 1 and 2 lights to use Surefire Scout tailcaps and remote switches. In particular, the Protac Rail Mount lights can now be used with the Surefire SR-D-IT and TNVC/Unity TAPS dual switches, which are commonly paired with laser/illuminators like the ATPIAL and DBAL series. One of the main flaws of the Protac Rail Mount lights is also addressed: Streamlight did not design a front battery spring into the light, so under recoil the battery can come off the front contact and cause the light to flicker or blink. To help prevent this behavior the adapter includes a removable buffer o-ring on the front face which ensures the battery cannot move rearward while firing.