Ultradyne USA MERCURY Brake, 5.56MM/223REM, Fits AR-15s with 1/2X28 Threads, Black, 3.1 oz., 416 Stainless Steel, Includes Shrouded Timing Nut UD10670 MERCURY has been meticulously engineered for the .223/5.56 caliber to provide superior recoil reduction and neutralize muzzle rise with a smooth" balanced release of blast energy. Progressive 90 side ports vent combusting gases at intervals that spread out the recoil impulse for efficient" accurate follow-up shots. Like all Ultradyne muzzle devices" a timing nut is included for easy installation.