The Close Quarter Battle Receiver (CQBR) is a replacement upper receiver for the M4A1 Carbine, developed by the US Navy. The CQBR features a 10.3 in (262 mm) length barrel (similar to the Colt Commando short-barreled M16 variants of the past) which makes the weapon significantly more compact, thus making it easier to use in, and around, vehicles and in tight, confined spaces. The overall length of the upper receiver is 19.25 inches (489 mm). With the stock retracted, the overall length of the weapon is 26.75 inches (679.4 mm). Those created as complete rifles (rather than only upper receivers) are type designated as MK 18 MOD 0/1. Special forces units utilize the CQBR for scenarios such as VIP protection, urban warfare, and other close quarters battle (CQB) situations. Its former preliminary National Stock Number (NSN) had been 1005-LL-L99-5996. A complete CQBR-equipped carbine now has the NSN 1005-01-527-2288. Background The M4 carbine and M16 are not ideally suited for all missions due to their relatively long barrels, but the modularity of AR pattern rifles allows the operator to easily replace the upper receiver. One of two proposed special mission receivers that were planned for inclusion into the SOPMOD Block II kit, the CQBR has taken off on its own. Like the proposed Special Purpose Receiver, the Close Quarters Battle Receiver has been more or less taken on by the Naval Surface Warfare Center, Crane Division (often referred to as NSWC-Crane or just "Crane") as its own project following the CQBR's removal from the SOPMOD program. Just as the Special Purpose Receiver morphed into the Special Purpose Rifle, and was type-classified as Mk 12 MOD 0/1, the complete CQBR-equipped carbine has been type-classified as the Mk 18 MOD 0, or the Mk 18 MOD 1 with a sightless gas block and full-length accessory rail kit. The purpose of the CQBR is to provide operators with a weapon of submachine gun size, but firing an intermediate cartridge, for scenarios such as VIP protection, urban warfare, and other close quarters battle (CQB) situations. The CQBR is designed to provide improvement over previous AR-15/M16-type weapons in this category. The CQBR is usually issued as a complete weapon system, and not just an upper receiver. The CQBR was once only available to Naval Special Warfare units (and by extension, other US Special Operations Forces units), but the Mk 18 MOD 0 has become general issue