MK18 mod 1 - Modified M4 with 10-inch barrel. Used by MARSOC units.

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. 

 

Specifications:

• Upper and lower receivers: The lower receiver is a standard M4A1 lower receiver sometimes modified with the addition of a CQD RSM sling adapter plate. However, many CQBR rifles also use surplus M16A1 lower receivers. Internally, the CQBR also takes advantage of a larger gas port and modified bolt carrier and buffer assembly. The gas port has been opened to 0.070 in (1.8 mm). A number of Lewis Machine & Tool 10.5" (266.7 mm) upper receivers have also been reported to have been purchased for the CQBR program. These upper receivers feature a gas system optimized for short barrel use.

• Sights: Initially, detachable carrying handles cut down so that only the rear sight assembly was used. However, most of these have been replaced with the similar, commercially-made Lewis Machine & Tool adjustable rear sight. Also used are several types of reflex and magnified optics, most commonly seen is the Aimpoint CompM2 in a Wilcox Industries Corp. Picatinny (MIL-STD-1913) mount.

• Stock: A variety of M4 retractable buttstocks are used on a standard 4-position receiver extension, including the standard "CAR" stock. Most often seen is the SOPMOD stock, often referred to as the "Crane stock", created by Dave Armstrong of NSWC-Crane. The angled stock offers better cheekweld as well as providing two storage compartments for spare batteries. The stock must be removed from the weapon to access the storage compartments. The initial run of stocks was made by NSWC-Crane from glass fiber polymer. Because of this, they were somewhat fragile and did not always properly lock into the receiver extension. A rubber band was used to secure the latch so it would not slide out of the desired position. The current L7LA2B SOPMOD stocks are made by Lewis Machine & Tools and have none of these problems.

• Barrel: The CQBR uses a 1:7 in (178 mm) twist M4 barrel that has been modified in length to 10.3 in (262 mm). The diameter of the barrel under the handguards is 1.17 in (29.7 mm). The KAC M4-QD flash suppressor is fitted, allowing use of the KAC QDSS-NT4 suppressor (National Stock Number 1005-01-437-0324). Although it has a bayonet lug, the CQBR is not meant to be fitted with a bayonet.

• Handguards: The standard handguard for the CQBR is the Knight's Armament Company Rail Interface System (M4 Carbine RIS; National Stock Number 1005-01-416-1089), which is designed for a variety of barrel profiles and allows for the use of any MIL-STD-1913 tactical accessories such as theAN/PEQ-2 infrared pointer, SureFire flashlights, or a vertical forward grip.

     The handguard manufacturer has since changed from KAC to Daniel Defense as the primary                contractor for the MK18 RIS, NSN # 1005-01-548-1385.

• Ammunition: The CQBR is designed to use standard 5.56×45mm NATO 62-grain SS109/M855 FMJ and M856 Tracer ammunition. However, due to the short barrel, the heavier 77-grain Mk 262 cartridge is preferred. There are no plans to rechamber the weapon in any additional calibers.

The purpose of the CQBR remains to provide operators with a weapon of submachine gun size,     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, but the Mk 18 Mod 0 has become general issue for Visit, Board, Search, and Seizure (VBSS) missions and, as of 2006, for NCISagents deploying to active combat zones.^{[citation needed]} 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.

The short 10.3 in (262 mm) barrel length requires special modifications to reliably function. The gas port is opened from 0.062 to 0.070 in (1.6 to 1.8 mm). 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 (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.