CAR-15 XM177

Although the United States had already tested the CAR-15 Model 607 submachine gun in Vietnam, it had a number of shortcomings, including overwhelming muzzle blast and an overly complicated, unreliable collapsible stock. To address these problems, Colt came up with the XM177, which featured a number of improvements. The triangular collapsible stock was replaced with a much simpler two-position telescoping tubular aluminum buttstock, while the fragile improvised handguards of the Model 607 were replaced by reinforced round handguards. Each half of the round handguard is identical, simplifying logistics by not requiring a top/bottom or left/right pair. The Model 609 Commando has a forward assist, while the Model 610 Commando does not. A Model 610B with a four-position selector was available, but not used by the U.S. military. All versions are equipped with the 4.25-inch (108 mm) long moderator.

The XM177 uses a unique flash hider sometimes called a flash or sound moderator for its 10-inch (250 mm) barrel. This device is 4.2 inches (110 mm) long and was designed primarily at the request of the US military's MAC V SOG as a method to confuse the Viet Cong and NVA, as the moderator makes the weapon sound more like the AK and AKM rifles in use by the Viet Cong and NVA. This device reduces flash signature somewhat and alters the weapons sound signature, making the normally louder short barreled carbine sound like an AK/ AKM.

The Model 610 was classified as the XM177[1] but adopted by the Air Force as the GAU-5/A Submachine Gun (GAU = Gun, Aircraft, fully-assembled Unit). The Army purchased 2,815 Model 609 CAR-15 Commandos on June 28, 1966, which were officially designated Submachine Gun, 5.56 mm, XM177E1.[1] As part of the contract, Colt was supposed to supply each XM177E1 with seven 30-round magazines, but Colt was unable to build a reliable 30-round curved magazine that would fit in the M16 magazine well, so most XM177E1s were shipped with 20-round magazines. The exception was 5th Special Forces Group, who received a total of four early 30-round magazines. Colt completed delivery of the purchased XM177E1s in March 1967.

In 1967, in response to field testing, Colt lengthened the Commando's barrel from 10 to 11.5 inches (254 to 292 mm). The increased length reduced noise and muzzle flash, and allowed fitting of the Colt XM148 grenade launcher. A metal boss was added to the moderator for mounting of the XM148 and rifle grenades.[2] The chambers were chrome-plated. The Commandos with the longer barrels were called the Model 629 and Model 649. The Model 629 Commando has a forward assist; the Model 649 Commando does not.

In April 1967, the Army purchased 510 Colt 629 Commandos for use by troops assigned to the Military Assistance Command, Vietnam Studies and Observations Group (MACV-SOG), and designated them XM177E2.[1] Delivery was completed by the end of September 1967. The Air Force adopted a similar model without the forward assist feature as the GAU-5A/A. Sources debate whether or not this was a Colt Model 630 or 649.[1] According to John Plaster and other sources, the lack of 30-round magazines continued to be problematic and SOG operators resorted to pooling their personal resources and purchasing the larger capacity magazines on the civilian U.S. market.[3] Problems with range, accuracy, barrel fouling, and usage of tracer bullets continued to plague the XM177 series, but Colt estimated that it would take a six-month $400,000 program to do a complete ballistic and kinematic study. There were also recommendations for a 29-month $635,000 research and development program. Both recommendations were declined by the U.S. military as American ground force involvement in the Vietnam War was gradually winding down. Production of the CAR-15 Commando ended in 1970.

It is rumored that the People's Army of Vietnam's M-18 was designed based on XM177 carbines seized from dead or wounded American and South Vietnamese soldiers in the Vietnam War.[4][5] They were first seen in public in 2010,[6] made by Z111 Factory.[7]

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  USAF Staff Sergeant with a GAU-5/A converted to a GUU-5/P and fitted with a Blank Firing Adaptor (BFA) for firing blanks.
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  USAF Combat Control Team (CCT) member with a GAU-5A/A. The GAU-5A/A can be distinguished from the GAU-5/A by the 1.5 inches (38 mm) of barrel between the moderator and the front sight block.
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  A USAF security policeman aims his GAU-5/A during a live-fire demonstration, part of Operation Desert Shield.