| The Close Quarters Battle Receiver (CQBR) is a replacement upper receiver for the M4A1 Carbine. The CQBR upgrades the M4 with a very short 10.3 inch (260mm) barrel, making it the modern equivalent of the Vietnam-era Colt Commando short barrel M16 variant. Originally intended as a component of the SOPMOD Block II kit, the CQBR is usually issued as a complete rifle, rather than as just an upper receiver. As a complete weapon, the CQBR is classified as the Mk18 Mod 0. The CQBR is intended for missions that the M4 and M16 are not suitable for, such as VIP protection, urban warfare and other Close Quarters Battle (CQB) situations. It provides rifle-level firepower in a package the size of a submachine gun. The very short length of the weapon makes it handier to use indoors and in tight quarters. The short barrel requires several modifications to the weapon to reliably function. An enlarged gas port, upgraded gas ring and stronger extractor spring are all added. This CQBR is fitted with a tactical light with a pressure switch mounted on the vertical foregrip, a LMT backup rear sight and an Aimpoint Comp M2 red dot sight in a Larue cantilever mount. It holds 30 rounds of 5.56x45mm NATO ammunition in a standard M16 detachable box magazine, and is 666mm long with the stock retracted. | 
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