CQBR

Close Quarter Battle Receiver
Top: An M4A1 with SOPMOD package, including Rail Interface System and Trijicon 4× ACOG. The barrel length is 14.5 inches (370 mm). Bottom: An M4A1 with a Close Quarter Battle Receiver. The barrel length is 10.3 inches (260 mm).
Type	Carbine
Place of origin	United States
Service history
In service	2000-present
Used by	See M4 Carbine users
Wars	War in Afghanistan War in Iraq
Production history
Designer	Colt Defense
Designed	1999
Produced	2000-present
Number built	Approx. >10,000
Specifications (CQBR)
Weight	2.72 kg (6.00 lb)
Length	762 mm (30.0 in) stock extended 679.4 mm (26.75 in) stock retracted
Barrel length	262 mm (10.3 in)
Cartridge	5.56×45mm NATO
Action	Gas-operated, rotating bolt
Muzzle velocity	788 m/s (2,585 ft/s)
Effective firing range	500 m (547 yd)
Feed system	20 or 30-round detachable STANAG magazine
Sights	Iron sights

The Close Quarter Battle Receiver (CQBR) is a replacement upper receiver for the M4A1 Carbine, developed by the US Navy. The CQBR replaces the M4 with a barrel 10.3 in (262 mm) length, making it the modern equivalent of the Colt Commando short-barrel M16 variants of the past. This shorter barrel makes the weapon significantly more compact, which makes it easier to use in and around vehicles and in tight confined spaces. Special units such as commandos boarding ships and bodyguards for senior officers have found such shortened weapons very useful and use the CQBR.

Its preliminary was 1005-LL-L99-5996; however, a complete CQBR-equipped carbine now has the NSN 1005-01-527-2288. 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).

The M4 carbine and M16 are not ideally suited for all missions, so it was proposed that the modularity of the M16 series would allow a user to replace the upper receiver of an existing weapon with one more suitable to the task. 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 remains to provide operators with a weapon of submachine gun size, but firing a rifle 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 Forces units), but the Mk 18 Mod 0 has become general issue for Visit, Board, Search, and Seizure (VBSS) missions and, as of 2006, for NCIS agents deploying to active combat zones. The Mk 18 is also used by the Coast Guard's Tactical Law Enforcement Teams, Maritime Safety and Security Teams, and Maritime Security Response Team and the United States Navy's Explosive Ordnance Disposal (EOD) Operators. It is also used by Marine Force Recon's CQB operators, and is in most cases the standard weapons of choice for said operators where the range and accuracy of an M4 is not necessary.