This is also the mounting device for the KAC M110 suppressor. 3/4" -24, which is specific to KAC SR-25 family of barrels.
The KAC M110 Flash Hider is an effective, birdcage-style flash suppressor that is currently in use on the US Military’s M110 SASS rifle system. It comes complete with a 4-piece shim kit.
Manufacturer's Part Number: KM-25877