Knight’s Armament Company is aware of a compatibility issue with the 5.56 MQC and CRS PRT (part number 119354 and 121567) when mounted on the SR-15/16 CQB (11.5” barrel) with 10.75” length URX4 handguard. The locking nut used on these newly-released suppressors were lengthened to allow mounting of the KAC suppressor shrouds, and this increase to length can cause interference between the front of the handguard and the rear of the locking nut. On release, it was decided to standardize on the shroud-compatible locking nut in an effort to provide the market with the best overall compatibility.
While the increased length was accounted for in the new line of URX6 handguards with backward-compatibility with all other Mod 2 URX4 configurations, the 10.75” URX4 and 11.5” barrel combination has a high probability of interference between the provided locking nut of a 5.56 PRT suppressor and the front of the URX4 handguard.
Those that desire to use the 5.56 MCQ or CRS PRT on an 11.5” CQB with 10.75” URX4 and are experiencing contact between the handguard and locking nut have three options: shim the muzzle device two rotations out from flush (0.075”), replace the locking collar with a shorter version, or replace the 10.75” URX4 handguard. Recently produced upper receivers with the notch in the top rail are directly compatible with the URX6 handguard line. Notched upper receivers will be made available for those with non-notched versions that desire to upgrade to the URX6.
Updating to the URX6 will provide full compatibility with the PRT suppressor line and their respective suppressor shrouds.
Please notify customer service at [email protected] if intending to use one of these suppressors on an 11.5” CQB with a URX4 handguard so that we may provide the appropriate corrective action and information.
Knight’s Armament Company apologizes for this inconvenience.