Ordnance Notes -- by Bob Stoner GMCM(SW) Ret.

Colt XM177 5.56mm Submachine Gun

XM-177E2 SMG with an XM-148 40mm grenade launcher attached. When carried with a sling attached, one end of the sling went through the holes in the sliding buttstock of the XM-177E2 and the other end attached to the front sight base. This arrangement of the sling allowed the weapon to be carried easily and ready for immediate use. SMG with an XM-148 40mm grenade launcher attached.

The Colt AR-15 (military designation M-16) was introduced to combat in Viet Nam under "Project Agile" in 1961. These early M-16 rifles were used by special operations forces and were an unwelcome surprise to the Viet Cong guerillas and North Vietnamese Army (NVA) troops. The M-16 was light weight, fast firing, and had little recoil. reports back from the field were glowing to the point that Secretary of Defense Robert S. McNamara and his Pentagon "whiz kids" took notice. The M-16 was on its way to augmenting, and then replacing, the 7.62mm M-14 rifle in American service.

Back in Hartford, CT, Colt's Firearms was basking in the adulation that its new M-16 rifle was earning in the combat zone. In response to requests from special forces operators in the Republic of Viet Nam and a request for a survival rifle based on the M-16 for the USAF, Colt's undertook the development of a short carbine.

The first attempt by Colt's resulted in a smart-looking carbine that took the standard 39-inch long rifle and shortened the 20-inch barrel by 10 inches, shortened the triangular handguards to take the 10-inch barrel, and the took 3 inches off the buttstock. The buttstock itself was designed to telescope closed for a more compact package and extend for shooting. The flash suppressor remained the same as the standard 3-prong M-16 rifle. Colt's promoted the carbine as the prefect weapon for helicopter and armor crews or special forces operators. Colt's called this carbine the CAR-15 "Commando" submachine gun (SMG).

Note: Submachine gun (SMG) is Colt's terminology. Technically, the true SMG uses pistol ammuntion, but marketing jargon has tended to blur this distinction. The XM-177 series (and its M-4 successor) are actually short rifles that shoot rifle ammunition. They are carbines, but the former was officially listed as a submachine gun and the latter is listed as a carbine.

The CAR-15 Survival Rifle for the USAF was similar to the CAR-15 "Commando" but had additional modifications. The cut-down triangular handguards of the original CAR-15 "Commando" were made round (and thereby eliminated the right/left handguard problem); the pistol grip was shortened by approximately 1.5 inches; the barrel remained at 10 inches; the 3-prong flash suppressor was replaced with a cone-shaped one; and the buttstock became a fixed, tubular metal type. The overall length of the USAF version was 29 inches and it weighed 5.4 pounds with a loaded 20-round magazine.

The original CAR-15 "Commando" telescoping stock was a good idea, but the design was found to be weak and prone to breakage. The shortened triangular handguards were likewise prone to breakage, and the 3-prong flash suppressor was totally worthless. In bright sunshine, the muzzle flash from the 10-inch barrel was a fireball about three feet in diameter. At night, the muzzle flash was so bad that it not only pin-pointed the shooter but any other troops near him! The USAF decided not to buy the CAR-15 Survival Rifle, and so Colt's went back to the drawing board.

The first redesigned CAR-15 "Commando" SMG to see wide spread use in Viet Nam was the XM-177. The XM-177 used the round handguards of the USAF Survival Rifle and retained the 10-inch barrel. Colt's developed a 4.5-inch "sound and flash" suppressor to reduce the muzzle flash of the short barrel. The specialized suppressor had six longitudinal slots cut in the end (similar to the enclosed "bird cage" flash suppressor that had replaced the 3-prong unit on the M-16 rifle). The specialized suppressor had an inner and outer shell; the inner shell had holes drilled for the muzzle flash to enter the space formed between the inner and outer walls of the shells. The new noise and flash suppressor reduced the blast signature (sound decibel level) to below that of the M-16A1 rifle. The XM-177 also introduced a sliding metal buttstock and the bayonet lug was ground off the bottom of the sight base.

When the M-16 rifle added a forward assist feature (to assist closing the bolt) in late 1967 to become the M-16E1 (later M-16A1), the XM-177 followed suit to become the XM-177E1. The XM-177E1 retained the 10-inch barrel of its predecessor with the forward assist feature. Continued muzzle flash problems remained with the XM-177E1. By mid-1968, the definitive SMG of the XM-177 series was fielded -- the XM-177E2. The XM-177E2 had all the characteristics of the previous XM-177 and XM-177E1 SMGs except that the barrel was lengthened to 11.5 inches. A spacer was added behind the special noise and flash suppressor to permit firing of rifle grenades (although it is doubtful that this use was ever made). The XM-177 series SMGs were much sought after by users in Viet Nam, but they were always in short supply. Navy SEALs used them, but they remained supplements to the basic M-16A1 rifle.

After American involvement in Viet Nam ended, the XM-177 series continued to soldier on with the Navy's SEAL Teams. By the early 1980s they were well worn or worn-out. The XM-177 series was replaced by the M-4 5.56mm NATO Carbine. The M-4 uses the action of the M-16A2 rifle; uses a modified version of the XM-177E2 telescoping buttstock; uses the round handguards of the XM-177 series; and has a 14.5-inch barrel that uses the flash suppressor/compensator of the M-16A2 rifle. Like the 20-inch barrel of the M-16A2 rifle, the M-4's barrel is cut to accept the standard M-203 40mm grenade launcher. The M-4 Carbine is current issue to Navy SEAL Teams and other special operations units.

Specifications of the XM-177E2 5.56mm SMG:
Length . . . . . 30 inches (stock retracted), 33 inches (stock extended)
Weight . . . . . 5.9 pounds
Barrel . . . . . 11.5 inches
Caliber . . . . 5.56x45mm NATO (.223 Remington)
Feed . . . . . 20 or 30 round magazine
Velocity . . . 2,995 feet per second
Rate . . . . . 750 rounds per minute


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2002 Bob Stoner