Editing Close Quarters Battle Receiver (section)

==Background==
The [[M4 carbine]] and [[M16 rifle|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 [[Mk 12 Special Purpose Rifle|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 Special Purpose Rifle|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 [[Executive protection|VIP protection]], [[urban warfare]], and other [[Close Quarters Battle|close quarters battle]] (CQB) situations.  The CQBR is designed to provide improvement over previous [[AR-15]]/[[M16 rifle|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 [[U.S. Naval Special Warfare Command|Naval Special Warfare units]] (and by extension, other [[United States special operations forces|US Special Operations Forces units]]), but the Mk 18 MOD 0 has become general issue for [[Visit, board, search, and seizure|Visit, Board, Search, and Seizure (VBSS)]] missions and, as of 2006, for [[Naval Criminal Investigative Service|NCIS]] agents deploying to active combat zones.{{Citation needed|date=August 2008}} The Mk 18 is also used by the [[United States Coast Guard|Coast Guard's]] [[Maritime Law Enforcement Specialist]]s within both [[Deployable Specialized Forces]] and [[Law Enforcement Detachments|Tactical Law Enforcement Teams]], as well as the [[Explosive ordnance disposal (United States Navy)|United States Navy's Explosive Ordnance Disposal (EOD) Operators]]. It is also used by both [[United States Marine Corps Force Reconnaissance|Marine Force Recon Teams]] and [[Critical skills operator|Critical Skills operators]] from [[United States Marine Forces Special Operations Command|MARSOC]], and is in most cases the standard weapons of choice for said operators where the range of an M4 is not necessary.

The short 10.3 in (262mm) barrel length requires special modifications to function reliably. The gas port is opened from 0.062 to 0.070 in (1.6 to 1.8mm). A one-piece McFarland gas ring replaces the three-piece gas ring set. The standard four-coil extractor spring is replaced with a [[Commercial off-the-shelf|commercial off-the-shelf (COTS)]] five-coil spring. An [[O-ring]] surrounds the extractor spring. The standard M4 flash hider has been replaced with the M4QD flash hider for suppressor compatibility.