Close Quarters Battle Receiver The Close Quarter Battle Receiver ( CQBR ) is a replacement upper receiver for the M4A1 Carbine . 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 . Its preliminary National Stock Number 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 ) . Background . 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 . 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 , 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 Team s , 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 ( 0 . 16 to 0 . 18 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 . Specifications . The handguard manufacturer has since changed from KAC to Daniel Defense as the primary contractor for the MK18 RIS , NSN # 1005 - 01 - 548 - 1385 .