XM177/E1/E2 5.56mm Submachine Gun
GAU-5/A and GAU-5A/A 5.56mm Submachine Gun

The XM177 and GAU-5/A series of submachine guns are modified version of the M16A1 5.56mm rifle, differing most notably in that they have a shorter barrel, a different type of flash suppressor, and a telescoping butt stock. The GAU-5/A is identical to the XM177, while the GAU-5A/A differs from the XM177E2 only in the lack of the bolt forward assist feature. A gas-operated and air-cooled weapon fed from a 20-round magazine, the XM177E2 can be fired from the shoulder or the hip. Semi-automatic or full-automatic fire can be delivered by the positioning of a selector lever on the left side of the lower receiver directly above the pistol grip.

The weapons are designated as submachine guns. The official Department of Defense definition of submachine gun as of 6 March 2009 was: A weapon fired from the shoulder or hip, with a barrel 8.5 to 11 inches long, using small arms ammunition and capable of semi or full automatic fire. This is different from the common definition that requires the weapon use pistol type ammunition.

The main components of the XM177E2 are the upper and lower receiver, bolt carrier group, barrel and barrel extension, trigger and hammer assemblies, handguard, sights, flash suppressor, and butt stock. Air-cooled, the 11.5 inch barrel is surrounded for the major portion of its length by a heat-resistant fiberglass material that acts as a handguard. The two-position butt stock is extended or closed by means of a release lever and lockpin. Similar to the flash suppressor of the M16A1, the modified suppressor of the XM177E2 is smoothly cylindrical, gas vents being machined in the cylinder near the front end. Both the front and rear sights are adjustable.

Performance characteristics of the XM177E2 are identical to those of the M16A1 rifle in all important respects. Muzzle velocity, usually measured in test fixtures and therefore somewhat of an approximation, drops about 25 feet per second for each inch of barrel length reduction. Muzzle velocity of the M16A1 is approximately 3,150 feet per second, but that of the XM177E2 is only 2,770, both velocities being achieved with the M193 55-grain ball cartridge. The cyclic rate of fire of the M16A1 is from 700 to 800 rounds per minute, not appreciably different from the cyclic rate of 650 to 900 of the XM177E2. Although a shorter barrel and the resultant lower velocity would change the midrange trajectory to a degree and the shorter sight radius would make precision aiming slightly more difficult, such differences are negligible.

During 1965 and 1966 10 5.56mm submachine guns, along with a number of other small arms weapons systems, were subjected to an extensive engineering test as part of the Small Arms Weapon Systems (SAWS) trials. The submachine guns supplied were identified variously as the CAR-15 or C-SMG at the time of the test. Colt Automatic Rifle-15 (CAR-15) was a term used by Colt to market an entire family of weapons derived from the Armalite AR-15, which had been the basis for the M16 rifle. The CAR-15 family included 2 rifles (which had been designated by the US military as the XM16 and XM16E1), 2 heavy barreled rifles (including one with a belt feed system), a carbine, a submachine gun, and a survival rifle. The usage of the term CAR-15 only for the submachine gun model during the SAWS trials led to the term being associated with short barreled members of the family despite the fact that the term applied broadly to Colt's commercial offerings. A small number of CAR-15 submachine guns, known also as Colt Model 607s, were acquired by the US Navy for use by SEAL teams, seeing limited usage in Vietnam.

After the engineering test, a modified flash suppressor and a new buffer design were submitted for an engineer design test and the weapon was designated XM177E1. The XM177 designation existed largely on paper, being applied to weapons without the bolt forward assist feature, similar to the difference between the XM16/M16 and the XM16E1/M16A1. The version without the forward assist was adopted by the US Air Force as the GAU-5/A. The Aeronautical and Support Equipment Type Designation System (ASETDS) did not have a personal weapon category at the time, and so the weapon was confusing designated in the Airborne Gun (GA) category.

A number of other product-improved components were then subsequently incorporated, most notably the extension of the barrel from 10 inches to 11.5 inches, and the designation of the weapon was changed to XM177E2. The US Air Force eventually adopted a similar weapon, again without the forward assist feature, as the GAU-5A/A. No US Army designation was assigned to this model. In March 1967, USATECOM, concurred in a recommendation from USAMC to type classify the weapon for temperate zone use but withheld comment on the suitability of the XM177E2 until a test of the product improvements was conducted.

Of related interest in the development of the XM177E2 submachine gun as of 1968 was the introduction of a new extruded-grain cartridge propellant identified as IMR-8208M to be used in the loading of M193 and M196 cartridges. The M193 and M196 cartridges were the standard ball and tracer cartridges, respectively, for the XM177E2 as well as the M16A1 rifle. In addition, substantial quantities of M196 cartridges were at that time being loaded with ball propellant, a projectile and propellant combination not previously evaluated in either CAR-15 submachine gun or XM177E2 weapons. By USATECOM direction, the new projectile-propellant combinations of M193 and M196 cartridges were to be tested simultaneously with the testing of the product improved version of the submachine gun during early 1968.

Tests in June 1968 showed that the XM177E2 submachine gun as designed was incompatible with the spectrum of ammunition as investigated in the testing, especially in those areas of performance affected by the buffer, and noise-flash suppressor. Both the XM177E1 and XM177E2 weapons gave unsatisfactorily high malfunction rates in the low temperature fouling test, and both weapons demonstrated more severe fouling in the operating mechanism with ball propellant than the IMR 8208M propellant. The XM177E2 weapons with chrome-plated chambers were superior to XM177E1 weapons with regard to failure-to-extract stoppages.

The tests also came to various conclusions about certain components and accessories. The delrin charging-handle latches on the XM177E2 weapons were inferior to the latches on XM177E1 weapons because of structural failure at -658o Fahrenheit. Within the scope of the test, no advantages in corrosion resistance were demonstrated for the shot-peened receiver, nylon-coated buttstock and release lever, and cadmium-plated slip ring spring of the XM177E2 weapons. The XM148 launcher spacer and the increased barrel length of the XM177E2 weapon permitted assembly of the attachment of the launcher to the XM177E2, which was not possible with the XM177E1. The angled slip ring on the XM177E2 weapons (no structural failures) proved superior to the flat slip rings on the XM177E1 weapons with respect to ease of assembly and disassembly on the hand guard.

The recommendations from the 1968 tests were that further development of the XM177E2 submachine gun buffer and noise-flash suppressor be accomplished; the delrin charging-handle latch be considered unacceptable; and the remaining product improvements under test be considered suitable for the XM177E2 submachine gun and, as appropriate, the M16A1 rifle. The angled slip ring was later incorporated into the M16A1E1 design in the 1980s.

The XM177 series was deployed to Vietnam being used by a variety of units, including US Army Military Police and US Air Force Combat Security Police (primarily by dog handlers) and US Army Scout Dog and Combat Tracker units. The weapons saw limited usage in various regular infantry units and unconventional warfare units.

What were later termed special operations forces continued to use the weapons for a period after, but these were gradually replaced first by the commercial Colt Model 653 carbine (also derived from the M16A1), and later by a number of commercial carbines based on the M16A2 and the standardized M4 carbine.

The US Air Force continued to use GAU-5/A and GAU-5A/A types longest after the end of the conflict in Vietnam. Programs to update the weapons led to number of different configurations similar to commercial Colt Model 653 carbines (also derived from the M16A1), which were in use by special operations forces. Further upgrades saw the weapons brought up to a standard close to that of the M4 and M4A1 carbines introduced in the 1990s. A Miscellaneous Gun category (GU) had been added to the ASETDS, and the weapons of the latter type were designated GUU-5/P. These often included a number of components held over from the original GAU-5 types. In December 2002, the USAF began to take delivery of the M4 carbine. By 2006, the M4 had replaced all daily use GAU/GUU types still in inventory for Security Forces, US Air Force Special Operations Forces and select Group A/B AFSCs utilizing those weapons.

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