The Arisaka Tailcap Adapter is designed to enable Streamlight Protac rail-mount HL-X and rail-mount HL-X Pro lights to utilize Surefire Scout tailcaps and remote switches. This adaptation allows these Protac rail-mount lights to be compatible with Surefire SR-D-IT and Unity AXON dual switches, commonly used alongside laser/illuminators such as the ATPIAL and DBAL series. A significant issue addressed by this adapter is a design flaw in the Protac rail-mount lights: the absence of a front battery spring. During recoil, this deficiency can cause the battery to lose contact with the front terminal, leading to flickering or blinking of the light. To mitigate this issue, the adapter includes two different O-rings, one for CR123 cells and another for 18650 cells. These O-rings are placed on the back end of the battery before screwing on the adapter. By securely holding the battery in place, the O-rings prevent it from shifting rearward under recoil, ensuring consistent contact and reliable performance during use.