XM177 — MACV-SOG’s Experimental 5.56mm

September 30th, 2025

6 minute read

By Robert A. Sadowski Posted in #History

Military Assistance Command, Vietnam — Studies and Observations Group (or MACV-SOG, for short) was a highly classified U.S. special operations unit that conducted covert operations before and during the Vietnam War.

Image of U.S. Navy SEALs in 1985 training with CAR-15 rifles, also identified as XM177 carbines. The rifles are shown with collapsible aluminum stocks extended, short barrels fitted with moderator muzzle devices, and standard 20- and 30-round 5.56mm magazines. Several weapons are lined up alongside training gear such as helmets, load-bearing vests, and web belts. Lighting and shadows suggest an outdoor or shipboard training space, typical for SEAL field drills during the 1980s. The XM177 carbines appear compact compared to the full-length M16A1 rifles, illustrating their advantage in close-quarters combat. This description includes keywords such as XM177, CAR-15, SEAL training, compact 5.56 carbine, collapsible stock, and Vietnam-era rifle variant for accessibility and SEO optimization. — Members of SEAL Team 4 participate in a training exercise. The sailor in the lead is armed with a CAR-15 assault rifle chambered in 5.56×45 NATO. Image: JO1 Peter D. Sundberg/U.S. Navy

This elite team was a mix of Special Operations forces and CIA guys who performed clandestine operations while sneaking around the jungles. Their tactics were as unconventional as their weapons, and there was no area of Southeast Asia that was out of their area of influence — from North Vietnam and South Vietnam to Laos and Cambodia. We called them advisors. They were the ones that did the work that needed to be done.

Mission-Specific Weapons

The standard-issue M16A1 just didn’t fit the profile of their special missions. Plus, because they fought North Vietnam troops in Cambodia and Laos where they weren’t authorized to be, they needed weapons that weren’t normally identifiable.

The image shows a U.S. Air Force combat control team during a 1979 training exercise, each operator carrying an XM177 Colt Commando CAR-15 rifle. The carbines are equipped with short 10–11.5 inch barrels, long moderator muzzle devices, and two-position collapsible aluminum stocks. Standard 20-round 5.56mm magazines are visible in several rifles, with some personnel carrying gear pouches and radios. The team is arranged in a tactical formation on open terrain, suggesting a field maneuver or insertion drill. Their uniforms are olive drab with standard issue web gear of the late 1970s. This description includes optimized terms such as XM177, Colt Commando, CAR-15, Air Force combat control, 5.56mm carbine, and 1979 training exercise for accessibility and search indexing. — Two airmen in a combat control team set marker panels around a drop zone during training in 1979 at Pope AFB. They are armed with XM177 rifles Image: Sgt Rozalyn Dorsey/U.S. Air Force

In some cases, they fielded the same weapons as their enemy, using captured AK-47s. Submachine guns were also carried since they were compact, lightweight and could pack a lot of ammo. Remember, these guys carried in what they needed. They were on their own in a hostile environment. There was no cavalry to come and save them if things went sideways. Suppressed pistols were common. Uzis and Swedish K submachine guns were also popular. This mix of weapons all offered plausible deniability that American troops were there.

There were other advantages. It’s easy to distinguish the sound of an AK from that of the M16. They both have a distinct sound. To a North Vietnamese guerrilla fighter, the sound of an AK meant friendly fire.

The photo depicts a U.S. Air Force security police airman during Operation Desert Storm, aiming a Colt Commando CAR-15 Model 610 XM177 on a shooting range. The carbine features a 10-inch barrel, long moderator muzzle device, and a two-position collapsible aluminum stock. The weapon is loaded with a 20-round magazine, and the airman is in a standing firing position wearing desert camouflage uniform and protective gear. The range setting includes sand berms and target silhouettes in the background, typical of military live-fire training. This compact rifle, chambered in 5.56mm, was widely used for security and defense roles during the Gulf War. Keywords included: XM177, Colt Commando, CAR-15 Model 610, USAF security police, Desert Storm training, compact carbine. — An Air Force security policeman aims at a target in training during Operation Desert Shield. Image: Tech. Sgt. Marvin Lynchard/U.S. Air Force

Compact firepower was a must for these warriors. However, the problem with submachine guns was that most were chambered in 9mm, which gave them limited power and range. The solution was a carbine in a rifle chambering.

What resulted in 1967 was the 5.56mm XM177E1. While compact AR’s with collapsible stocks are extremely common these days, this configuration was revolutionary at that time. The XM177 was truly experimental.

XM177: Shorty M16, Commando Approved

The XM177 is a shortened version of the M16. The M16A1 was lightweight, had minimal recoil and offered impressive firepower, but — with a 20” barrel — was anything but compact.

U.S. Air Force combat controllers are pictured in a 1970s–1980s era training scenario, wading through a river while armed with XM177 rifles. The carbines are visibly compact, featuring moderator flash hiders, collapsible stocks, and standard-issue 20-round magazines. Each man holds his rifle above shoulder height to keep it dry while crossing. Their field gear includes helmets, web belts, and radios, marking them as specialized operators rather than regular infantry. The scene highlights the challenges of special operations mobility in waterborne environments. XM177, CAR-15, Colt Commando, combat control team, river navigation, special operations carbine. — Members of the Norton AFB combat control team hold their XM177 assault rifles out of the water as they swim to shore during an amphibious training mission in 1978. Image: Staff Sgt. Joseph F. Smith Jr./NARA

MACV-SOG needed to be quick on their feet in the deep jungle where they fought. Long, heavy weapons would be a liability. The main requirement for the XM177 was to be shorter and more compact. The M16A1’s 20” barrel was deep-sixed for a stubby 10” barrel with a “moderator” muzzle device screwed onto the end. A collapsing stock made the XM177 even more compact. The XM177 was basically the size and weight of a submachine gun, but with 5.56, rifle-caliber firepower. It outperformed any submachine gun in use at the time.

Designed for Jungle Warfare

As noted, the basis for the XM177 was the M16A1. The barrel, moderator and stock were the three features that were focused on and modified to create these shorty M16s. The selector switch still offered safe, semi and full-auto modes of fire.

The photo depicts a U.S. Air Force combat control team seated in an inflatable boat during a training exercise, each man armed with a GAU-5 variant of the XM177 carbine. The carbines are identifiable by their short barrels with moderator muzzle devices, collapsible aluminum stocks, and inserted 20-round 5.56mm magazines. Team members wear field uniforms, helmets, and tactical web gear, with radios and equipment visible inside the boat. The inflatable boat is in calm water, suggesting a river or coastal training area. The compact rifles are held at the ready, emphasizing their role in rapid-action maritime operations. GAU-5, XM177, CAR-15 Commando, USAF combat control, inflatable boat training, 5.56mm carbine. — Combat control team members, armed with XM177 assault rifles, maneuver down a river by an inflatable boat during training. Image: Staff Sgt. Ernest H. Sealing/NARA

The two-position collapsible stock was a new concept at the time. Collapsed, the weapon was compact and easy to transport. The buttstock, made of aluminum, shortened the overall length of the carbine to just 28”. The M16A1 as-issued had a length close to 39”. This was long before six-position AR stocks were designed to compensate for body armor, gear or body stature.

There was plenty of experimentation with the M16 during the Vietnam War, and shortening the barrel was one common modification. Since then, we’ve learned a lot about reducing the barrel on an AR platform and keeping the system reliable. Back then, it was trial and error.

Close-up image of the XM177 moderator (muzzle device) mounted on a short-barreled CAR-15/XM177 carbine, photographed for Rock Island. The metal moderator is fixed to the muzzle and shows a smooth cylindrical exterior with a small exit port, designed to reduce visible muzzle flash and slightly lower the sound signature. The description emphasizes that the device is not user-disassemblable, making field cleaning difficult and causing performance to degrade as carbon and debris build up. The photo’s framing highlights the interface between the barrel and moderator and includes part of the short barrel and front sight for context. This alt text uses keywords such as XM177, moderator, muzzle device, CAR-15, flash suppression, and fouling to aid discoverability. It’s written to inform screen reader users about both the appearance and the functional limitations of the original XM177 moderator. — The XM177 moderator could not be disassembled to be cleaned, and it became less effective when dirty. Image: Rock Island Auction Company

As noted, the XM177 used a 10” barrel. Of course, the direct gas impingement system needed to be shortened, as well. A long flash-hider with a small diameter exit port was added to the muzzle. During testing, the 10” barrel created a fireball out the muzzle every time it was fired, which wasn’t conducive to clandestine operations. The reworked gas system was reduced to compensate for the short barrel, and reliability was sketchy in the initial guns. The XM177 needed more work to be battle-ready.

Back to the Drawing Board

The result from more testing and modifications was the XM177E2, which used a longer 11.5” barrel and an improved moderator that reduced the noise signature and muzzle flash. The moderators worked, but they could not be disassembled like current suppressors. After extended use, the moderator started to clog up and was less effective. Interestingly, the sound proved to be more like an AK than an M16, which was important for guys running and gunning behind enemy lines.

The image shows a typical XM177 rifle, also known as the Colt Commando or CAR-15 variant, as discussed in the article. The carbine features a short 10-inch barrel with a long moderator muzzle device, designed to reduce flash and change the weapon’s sound signature. A two-position collapsible aluminum stock extends from the receiver, reducing the rifle’s length from nearly 39 inches down to about 28 inches. A slim 20-round 5.56mm magazine is inserted, although later versions often used 30-round magazines. The overall design is compact, lightweight, and clearly intended for close-quarters combat and jungle mobility. XM177, Colt Commando, CAR-15, compact 5.56mm rifle, MACV-SOG weapon, and Vietnam War carbine for accessibility and SEO purposes. — A typical XM177 rifle with moderator and collapsible stock, as fielded in the late 1960s. The carbine’s lightweight and portable design influenced the later M4 Carbine. Image: Rock Island Auction Company

The XM177E2 was issued in small numbers. It used the standard 20-round steel magazine that was issued during the war, while 30-round magazines were also employed. The E2 wasn’t perfect. The moderator caked up with debris, making it less effective and causing the cyclic rate to vary and causing bullets to yaw. Eventually, the bugs were ironed out.

Conclusion

More work could have been put into the MX177E2 to make the moderator easier to maintain and more effective, but the war in Vietnam was winding down and there was no funding to continue development. However, they were definitely a success in the bigger picture. The XM177 carbines allowed MACV-SOG to be quick on their feet and maneuverable in dense jungle environments, offering impressive firepower in a compact package.

The work and effort put into the XM177E2 was not forgotten, foreshadowing and setting the stage for the development of the M4 Carbine. The XM177E1 and E2 variants were the pioneers of the ultra-compact assault rifle, resulting in the military’s M4 Carbine. — a direct descendant of the innovative XM177.

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Robert A. Sadowski

Robert A. Sadowski has written about firearms and hunting for more than fifteen years. He has trained with some of the country’s finest firearm instructors in handguns, rifles/carbines, shotguns and long-range shooting. He is the author of numerous gun books, including 9MM — Guide to America's Most Popular Caliber, a #1 New Release on Amazon. He is a contributing editor to numerous gun-enthusiast magazines and websites, including Combat Handguns, Black Guns, Gun Tests, Gun Digest, Gun World, Ballistic, range365.com, SHOT Business, and others. He also edited Shooter’s Bible Guide to Firearms Assembly, Disassembly, and Cleaning; 50 Guns That Changed the World; and Gun Traders Guide.

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