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

Colt XM177 5.56mm Submachine Gun

A Colt XM177E2 submachine gun (SMG) with a 30-round magazine and an XM148 40mm grenade launcher attached. When carried with a sling, one end of the sling went through the holes in the sliding butt stock of the XM177E2 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. The Colt-designed XM148 40mm grenade launcher is described separately in Ordnance Notes. (Photo: Colt Firearms)

The Colt AR-15 (military designation M16) was introduced to combat in Viet Nam under "Project AGILE" in 1961. These early M16 rifles were used by special operations forces and were an unwelcome surprise to the Viet Cong guerillas and North Vietnamese Army (NVA) troops. The M16 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 M16 was on its way to augmenting, and then replacing, the 7.62mm M14 rifle in American service.

Back in Hartford, CT, Colt's Firearms, who was doing production of the rifle for Armalite, was basking in the adulation that the new M16 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 M16 for the USAF, Colt's undertook the development of a short carbine in 1964.

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 hand guards to take the 10-inch barrel, and the took 3 inches off the butt stock. The butt stock 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 M16 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 ammunition, but marketing jargon has tended to blur this distinction. The XM177 series (and its M4 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.

A Colt CAR-15 “Commando” SMG with a 20-round magazine. The rifle had a 10-inch barrel with Colt’s special sound and flash suppressor. It used the shortened triangular hand guards from the AR-15 with a special triangular rear cap and had a shortened, collapsible stock. The stock latch was located in the notch cutout of the butt plate. To extend the stock, the latch was pushed to the side and the stock pulled back. The upper part of the stock was supported on the buffer tube and a guide rail kept the stock from turning about the buffer tube. (Photo: AR15.com)

The CAR-15 Survival Rifle for the USAF was similar to the CAR-15 "Commando" but had additional modifications. The cut-down triangular hand guards of the original CAR-15 "Commando" were made round (and thereby eliminated the right/left hand guard 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 butt stock 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 hand guards 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 and received DoD approval for an initial buy of 2,815 guns for the USAF and US Army on June 28, 1966. September 1966 trials at Aberdeen Proving Grounds in Maryland resulted in the requirement for development of a new “sound and flash” suppressor to replace the 3-prong unit then used on the M16 rifle.

In January 1967, the “Commando” was type-classified as XM177 (USAF) and XM177E1 (US Army). The XM177 used the round hand guards 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 M16 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 M16A1 rifle. However, the new suppressor did have fouling problems that were never fully solved. The XM177 also introduced a sliding aluminum butt stock and the bayonet lug was ground off the bottom of the sight base.

The US Army was adamant about the forward assist (bolt closure) feature of the XM16E1. This rifle was designated XM177E1. The XM177E1 retained the 10-inch barrel of its predecessor with the forward assist feature. Continued muzzle flash and fouling problems remained with the XM177E1. By mid-1968, the definitive SMG of the XM177 series was fielded -- the XM177E2. The XM177E2 had all the characteristics of the previous XM177 and XM177E1 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 XM177 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 M16A1 rifle.

The M4 5.56mm NATO Carbine in its original configuration. Note the changes in the new collapsible plastic butt stock, M16A2 pistol grip, barrel machined to accept the M203 40mm grenade launcher, and reversion to the standard rifle flash suppressor. Since first issued, M4s have gone through modifications to add a detachable carrying handle and the Picatinny Arsenal Rail Integration System (RIS) for the mounting of advanced sights, target designators, and hand grip. (Photo: Bushmaster Firearms)

After American involvement in Viet Nam ended, the XM177-series continued to soldier on with the Navy's SEAL Teams. By the early 1980s they were well worn or worn-out. The XM177 series was replaced by the M4 5.56mm NATO Carbine. The M4 uses the action of the M16A2 rifle; uses a modified version of the XM177E2 telescoping butt stock; uses the round hand guards of the XM177 series; and has a 14.5-inch barrel that uses the combination flash suppressor and compensator of the M16A2 rifle. Like the 20-inch barrel of the M16A2 rifle, the M4's barrel is cut to accept the standard M203 40mm grenade launcher. The M4 Carbine is current issue to Navy SEAL Teams and other special operations units.

Specifications of the XM177E2 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 to 850 rounds per minute

© 2005 Bob Stoner R4