This product is not compatible with the Protac 2.0 Rail Mount Lights. 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 HL-X 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 two different o-rings (one for CR123 cells, one for 18650 cells) which can be placed on the back end of the battery before screwing on the adapter. The o-ring ensures the battery cannot move rearward while firing. This adapter requires the use of a Surefire UE or DS tailcap to function. It also does NOT work with the Rail Mount HL-X Laser model.Features : ,Diameter: 1.04";"Length: 0.49" additional length to light (plus tailcap), Weight: 0.6 oz The Arisaka Tailcap Adapter is CNC machined from 6061-T651 aluminum and MIL-A-8625 Type III hard coat anodized Made in the USA