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Early SAW: Colt's AR-15 Heavy Assault Rifle M1

A look at an early experimental Colt heavy barrel light machine gun design which foreshadowed things to come!

Early SAW: Colt's AR-15 Heavy Assault Rifle M1

Looking for additional lucrative military contracts Colt developed a family of small arms based off the AR-15 operating system in the early 1960s, including the AR-15 Heavy Assault Rifle M1 seen here.

Colt’s purchase of the AR-15 design from Armalite in 1959 would prove to be a huge success for this historic American company. But, it wasn’t an easy road to government and military contracts. The original selective-fire AR-15 Model 601 rifles produced for government sales were eventually successfully marketed to the US military, fielded for testing in Vietnam and later evolved into the M16 and XM16E1. This success was a boon to Colt, and accomplished despite the “not invented here” mentality of US Army Ordnance.

After thrashing the US Army’s darling M14, Colt didn’t rest on its laurels. They realized other new modern competitive designs could threaten their initial success. Keep in mind, Eugene Stoner had left ArmaLite and began working with Cadillac Gage on a modular weapon design. Wanting to maintain their momentum, Colt’s R&D team began developing a family of models in the early 1960s based off the original AR-15 operating system. These seven models (standard rifle, standard rifle with 40mm grenade launcher, survival rifle, 10-inch “submachine gun”, carbine, magazine-fed LMG and a belt-fed LMG) were then marketed to the US military. The Colt Mfg. AR-15 Heavy Assault Rifle M1, also known as the Colt AR-15 Model GX-5856 was developed as part of the original CAR-15 family of new designs. This 5.56mm rifle is the precursor to the M16 Light Machinegun and can be thought of as conceptually similar to the Soviet 7.62x39mm RPK squad auto in the role it was designed to fill.

Colt’s AR-15 Heavy Assault Rifle M1
Colt envisioned the AR-15 Heavy Assault Rifle M1 (also known as the Colt AR-15 Model GX-5856) to be a light support weapon similar to the Soviet RPK seen here.

Between 1964 and 1965, Colt Mfg. began an attempt to expand the AR-15 rifle outside that of a light infantry rifle/Assault Rifle. With the official US military adoption of AR-15 Models 603 and 604 (XM16E1 and M16 respectively), Colt believed they had a chance to break into the lucrative Light Machine Gun realm of weaponry. With this, the Model 606 series was born.

In February 1965, the U.S. Army undertook the Small Arms Weapons Systems trials (SAWS trials). Colt Mfg. responded by submitting the AR-15 Model 606A automatic rifle for trials. With this submittal as a candidate weapon, it would take on the Government/Experimental classification and be known as the GX-5856/Heavy Assault Rifle M1. The designation “Heavy Assault Rifle” indicating the barrel profile. Colt manufactured approximately 2,443 GX-5856/Heavy Assault Rifles. These rifles were given a serial number range of 750,000 to 752,443 (according to Colt Mfg.) The 606/Heavy Assault Rifle series of AR-15s were the first AR/M16 pattern rifles to be fitted with a heavy profile barrel. Rifling twist was 1 turn in 12 inches with a right hand twist.


As this was intended to fill the role of a light machine gun, it was fitted with a bipod. It used a proprietary bipod mounting system, which used the M2 bipod typically found on the M14E2 variant of the 7.62x51mm M14 Battle Rifle. This was mounted directly to the barrel. It also used a plastic, experimental buffering system (proprietary and unique to the 606). The rest of the 606s were essentially 603 and 604 rifles (depending on the variant), with the 606B having a 4-position (0-F-1-3) selector switch. This Foster E. Sturtevant designed selector switch and fire control system allowed the gunner to select between semi, burst and auto fire.


The 606 series, along with the Heavy Assault Rifles, had varying sub-category letters, differentiating the changes between one another. While all 606s ultimately had the same basic features (listed), they designed these LMG precursors for varying military branches. The Model 606 would feature no forward assist (see Auxiliary Bolt Closure Mechanism, F.E. Sturtevant, patent number 3,236,555) and was ultimately a heavy barrel, light support weapon variant of the U.S.A.F. (and later U.S. Navy Brown Water Patrol, along with U.S. Marines, circa 1966-1967) AR-15 Model 604/M16 rifle.

Colt’s AR-15 Heavy Assault Rifle M1
Colt manufactured approximately 2,443 GX-5856/Heavy Assault Rifles, but the design was not adopted by the US military.

The Model 606A would feature Foster E. Sturtevant's recently designed Auxiliary Bolt Closure Mechanism, designed for use by the U.S. Army. This was the rifle sent to U.S. Army SAWS trials, and was ultimately a light support weapon variant of the AR-15 Model 603/XM16E1. The 606B variant, which was ultimately the same rifle, featured a 4-position selector. The AR-15 Model 611, which was a Commercial/Export variant of the 606 rifle. This rifle would be designed and produced (in limited numbers) between 1966 and 1967. This would ultimately be the Export variant of the M16A1 LMG. Other firearms in the Commercial/Export line would be the Model 616, which was a forward assist delete variant of the rifle.

Ultimately, the 606 series Heavy Assault Rifle M1s didn't flourish. They rapidly overheated and the early gas impingement system (meaning the early, softer gas tubes of the 601-604/M16), coupled with the early white nylon buffer didn’t lend well to the sustained fire for which it was intended. While a decent overall concept (light, portable machine gun/automatic rifle), neither the M1 nor the experimental belt-fed Heavy Assault Rifle M2 weapons platforms were ever adopted. After the SAWS trials many were shipped off to military museums. Around 1990 100 of them were demilled at Anniston. These firearms did however lead to the M16 LMGs of later dates and helped pave the way for the L. James Sullivan Ultimax 100 LMG. So, while the Heavy Assault Rifle M1 and M2s were short lived projects, the concept would live on.

If you have any thoughts or comments on this article, we’d love to hear them. Email us at FirearmsNews@Outdoorsg.com.




About the Author:

Michelle Hamilton has a Bachelor’s degree in Criminal Justice/Homeland Security, is a serious student of military history, small arms design and manufacturing and is a competitive shooter.

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