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.