This is a Knights Armament Company Muzzle Brake known as the M4QD chambered in 556NATO, model 93048.
The KAC M4QD Flash Hider is an effective flash suppressor that also serves as a platform for attaching a Knight's Armament quick detach QDSS-NT4 sound suppressor. Comes complete with a 7-piece shim kit.
Features:
Steel construction
1/2X28 thread pitch
Compatible with KAC 5.56 QDSS-NT4
