Knight's Armament Company Upper Receiver Extension

The Knight's Armament Company Rail Adapter System II Upper Receiver Extension, or RAS II URX, is a free-floating rail adapter forend system for AR-15 type rifles.

The URX was created as an entry for the RIS II portion of the SOPMOD (Special Operations Peculiar Modification) Block II upgrade program. It was rejected because mounting a M203 grenade launcher prohibited free-floating of the rifle barrel. The follow-up URX II rectified this by accommodating a modular mount for the M203 that does not impinge on the rifle barrel in any manner. The URX series allows for the attachment of various rail-mounted devices, much like its Knight's Armament predecessors, the Rail Interface System (RIS), the Rail Adaptor System (RAS), and the Knight's Armament Company Free Floating Rail Adaptor System (FF RAS). Unlike them, the URX series differs by eliminating the use of a delta ring between the handguard and the upper receiver of the weapon, since itself becomes the barrel nut, which secures the barrel to the upper receiver. On late-model AR15s that feature a railed flat-top upper receiver, this effectively creates a single long upper rail running the entire length of the weapon, from the beginning of the receiver flat-top at the rear portion of the upper receiver all the way to the forward termination of the upper rail on the handguard. This unit is a free floating unit in that it never touches the barrel of the weapon (much like the Knight's Free Floating RAS). While the earlier rail systems by KAC could be installed easily by the end user, the URX requires specialized tools. Installation of the URX on an existing firearm requires it being sent to KAC or to a few select dealers.

The URX provides several advantages. It provides an uninterrupted rail and does not increase the height of the rail over the barrel, which may cause sighting problems for attached optics, unlike the ARMS Selective Integrated Rail (SIR) system. With the URX installation, the barrel is free-floated, allowing for increased accuracy. Due to increased contact with the upper receiver, increased heat transfer occurs, making the URX a better heat sink, reducing the chance of "cook off". Unlike other free-float systems available from LMT, Daniel Defense, Larue Tactical, or KAC itself, the lower can be removed for easy cleaning of the barrel. The lower half slides into rails in the upper half and is locked into position with 1/4"-20tpi hex nuts.

To mount an M203 on a firearm with a URX II rail system requires the URX II M203 rail (KAC Part number: 23184). The lower half of the URX II is removed. Then the M203 is locked into the M203 rail, which itself slides into the URX II rail and is secured with hex nuts. This allows the recoil of the M203 to be absorbed by the whole upper receiver assembly, not just the barrel itself.

The URX II is currently available in a carbine-length version that weights 8.6 oz., a mid-length version for use with an 11.375"-long gas system that weighs 12.7oz., a match version for rifles that weighs 14.4oz. (KAC Part number: 23258), and an SR-25 version. On the bottom and the sides of the carbine version are fifteen mounting slots. Odd-numbered slots are marked starting with 17 at the rear, going up to 31. The top has seventeen slots, marked T15 to T31. The mid-length version is made from cutting down a match version. It has twenty-two mounting slots on the sides and bottom and twenty-three on the top. The front of the top section has a notch cut out for fitting KAC's barrel-mounted folding flip-up sight. The markings are identical to the carbine version, except going up to 37. The match version has 31 slots on top and 29 slots on the other three sides with numbers going up to 45.