Advanced Combat Rifle Prototypes

4 ACR program Rifle (Matthew Moss)
The four rifles tested during the final phases of the ACR program, AAI, Colt, H&K, Steyr (Matthew Moss)

With so few primary or secondary sources on the ACR program available, this article relies heavily on the 1990 program summary report written by the US Army Armament Research, Development and Engineering Center (ARDEC). Over the next three articles we will examine the AAI, Colt, and Steyr ACR entries. The H&K entry was covered earlier – here.

In the early 1980s the US Army began looking into what form a replacement for the M16A1 might take. The Joint Service Small Arms Program undertook a study and found that in the long term a “technology base should be developed to support a significant improvement in capability.” This meant the US infantryman’s next rifle would move away from conventional ammunition and actions.

The Advanced Combat Rifle program began in January 1985, to foster the development and select a rifle that would give troops a distinct advantage on the battlefield. The project’s ambitious aim was to address the human shortcomings of the average soldier in the field. It was found that combat stress, fear and fatigue negatively impacted on a rifleman’s ability to hit a target once engaged in combat.

US Army Future Sldier 1959
US Army’s 1959 Future Soldier Concept, with M14 (LIFE)

The ACR program sought to compensate for this by maximising hit probability when engaging fleeting targets in battlefield conditions. The program called for:

“an enhancement in hit probability of at least 100 percent at combat ranges over the baseline performance of the M16A2 rifle… at extended ranges, the improvement required will be considerably greater than 100 percent. The weapon will be expected to enable the rifleman to detect rargets at ranges greater than 400m in offensive action and at least 1000m during the conduct of the defense.”

This was a tall order, calling for a weapon with an optic and the ability to engage long range targets, which were envisaged to be wearing body armour, but also engage fleeting targets. But the ACR program was far from the US military’ first ambitious small arms project.

The ACR program built on the Special Purpose Individual Weapon (SPIW) program that had been established in the 1950s to produce the next generation of infantry weapon capable of firing busts of flechettes at extremely high rates of fire to improve hit probability. This task proved technologically insurmountable during the 1950s and 60s. The SPIW program ended in the late 1960s, but was followed in the early 1970s by the Future Rifle Program which also proved a failure. Despite these failures the programs helped to push small arms technological boundaries.

In the video below, produced by ARDEC for the US Department of Defence, we see the ACR program briefly explained and the various rifles introduced with some firing footage:

The approaches used by the companies that entered the ACR competition varied greatly. Ranging from rifles with complex buffer systems to weapons that fired duplex and flechette rounds.

The first Request for Proposals was released in September 1985, with six companies successfully submitting proposals. These companies, which were awarded a Phase I contract, were: AAI Corporation, Ares Incorporated, Colt’s Manufacturing Company, Heckler & Koch (H&K), McDonnell Douglas Helicopter Systems (MDHS), and Steyr Mannlicher. In 1987, following Phase I of the program the entries from Ares and MDHS were dropped due to ‘hardware immaturity’ – the concepts were not developed enough. Both companies appealed and were allowed to enter designs to Phase II but neither was sufficiently developed to compete in the later trials and testing.

Each company approached the program’s aims in a different way. Two utilised flechette technology, which had been developed during the SPIW program, with the AAI using a brass cased round while the Steyr ACR used a polymer case. Colt’s entry used a duplex round with two projectiles stacked in the cartridge case. Heckler & Koch’s G11 used a complex buffer system to reduce felt recoil after firing a ‘hyper burst’ of three rounds at ~2,000 rounds per minute.

Lets take a look at the individual weapon systems:

AAI Corporation

AAI Corp ACR rifle
AAI Corporation’s ACR entry (Matthew Moss)

Check out Vic’s video on the AAI ACR and an article with further detail here

AAI Corporation had long been involved in advanced firearms design, taking part in the US military’s previous programs. The AAI ACR was an evolution of the earlier SFR/XM19 rifle. AAI’s ACR was gas-operated, feeding from a 30 round box magazine and fired a brass cased flechette round in either single shot or a high cyclic rate three round bust.

It fired from a closed bolt and used a muzzle device to reduce muzzle climb during burst fire. AAI developed a polymer saboted steel 10.2 gr flechette which fitted within a standard M855 brass case. As a result the rifle uses a proprietary magazine to avoid the accidental chambering of conventional 5.56x45mm rounds. These rounds had a muzzle velocity of 4,600 ft/s.

Diagram showing AAI's saboted flechette round inside a 5.56x45mm M855 case (US Army)
Diagram showing AAI’s saboted flechette round inside a 5.56x45mm M855 case (US Army)

The rifle was designed to mount a quick detachable 4x optic and had a white-highlighted shotgun-style rib sight along the barrel to aid in snap shooting. The rifle was one of the longest entries with an overall length of 40 inches or 101.6cm.

According to the ACR program summary the AAI entry proved to be a “mature design which performed in a reliable fashion” during the field trials.

Specifications (From ACR Program Summary):

Length: 40 inches / 101.6cm
Weight: 9.39 lbs / 4.26kg
Sights: Iron or 4x optic
Action: Gas operated
Calibre: 5.56mm brass cased flechette
Feed: 30-round box magazine

Colt’s Manufacturing Company

Colt ACR rifle submission
Colt’s ACR entry (Matthew Moss)

Check out Vic’s video on the Colt ACR and an article with further detail here

Colt’s entry was perhaps the most conventional of the designs submitted. Based on the rifle the program sought to replace. Colt’s ACR was essentially an improved M16, which fired both conventional 5.56x45mm ammunition as well as a new 5.56mm duplex round. While the duplex round increase hit probability at shorter ranges, it impacted long range accuracy requiring the additional use of conventional M855 rounds.

It incorporated a variety of improvements including a new oil/spring hydraulic buffer to mitigate recoil. A reshaped pistol grip and a hand guard which mounted a sighting rib for snap shooting. The weapon had a flat-top upper receiver which was railed so a 3.5x optic (an early ECLAN) or a more conventional sight/carrying handle could be fitted.

Colt's 5.56mm Duplex round (US Army)
Colt’s 5.56mm Duplex round (US Army)

The rifle’s collapsible six position telescopic butt stock was an improved version of that offered with Colt’s carbines. When at full extension the Colt ACR was the longest rifle tested, at 40.6 inch or 103 cm long. A distinctive proprietary muzzle brake compensator designed by Knight’s Armament was also added.

During testing one of the duplex rounds was not properly seated inside the cartridge case and when fired became lodged in the barrel and during the course of fire and the weapon’s barrel blew when another round was fired. This was addressed by a slightly larger propellant charge. Some of the features developed for the ACR entry were later employed in the M16A3 and later A4.

Specifications (From ACR Program Summary):

Length: 40.6 inches / 103cm (extended) and 36.7 inches / 93.2cm (collapsed)
Weight: 10.3 lbs / 4.67kg
Sights: iron or 3.5x optic
Action: Direct gas impingement
Calibre: 5.56mm duplex round & M855 ball
Feed: 30-round box magazine

Heckler & Koch

Heckler & Koch's G11K2, ACR entry
Heckler & Koch’s G11K2, ACR submission (Matthew Moss)

Check out our full article on the G11 and our introductory video showing two examples of the weapon partially disassembled here

Heckler & Koch had been involved in the West German Bundeswher’s attempts to create a next generation infantry weapon. The result was the G11, the G11 had been in development since the late 1960s and by the time it was entered into the ACR program it had evolved into a complex weapon unlike any other. Utilising a gas-operated, rotary breech to fire self-contained caseless 4.73×33mm projectiles. The rotary breech chamber was introduced as a means of clearing misfired, broken or defective cartridges from the enclosed system. Simplicity was sacrificed to achieve reliability.

The US had awarded H&K a contract to develop caseless ammunition, optics and the salvo concept in 1982. With the G11 in development they then entered the ACR program. The bullpup H&K was the shortest of the weapons entered into the trial at 29.5 inches / 74.9cm. It was equipped by a variable 1-3.5x optic designed by Swarovski.

g11round5
Diagram showing the composition of the G11’s telescoped caseless ammunition (source)

The revolutionary ammunition was developed by Dynamit-Nobel AG.
The 4.73x33mm, 51gr round which was finalised comprised of a solid propellant material body which encased a primer, booster, projectile and a plastic nose cap. Dynamit-Nobel developed the High Ignition Temperature Propellant (HITP) in an effort to prevent accidental ignition (cook-off) of the ammunition’s outer propellant body. The ACR program summary notes that “the majority of the malfunctions/stoppages experience in the field experiment were ammunition related.”

The H&K ACR fed from a single stack, horizontally orientated 45-round magazine which sat above the barrel.

The G11 used a counter-recoil buffer mechanism to allow high rates of burst fire – hyper burst. When firing three round bursts the weapon could send the rounds downrange at a rate of ~2,000 rounds per minute, only when the last round has left the barrel does the barrel and action begin to recoil inside the stock along a central guide. The recoil impulse was then quite strong. When in sustained fire the rate of fire is closer to ~460 per minute. A more detailed explanation of the G11’s action can be found here.

Specifications (From ACR Program Summary):

Length: 29.5 inches / 74.9cm
Weight: 9.15 lbs / 4.15kg
Sights: Variable 1-3.5x optic
Action: gas-operated, rotary breech
Calibre: 4.92mm caseless
Feed: 45-round single stack magazine

Steyr-Mannlicher

Steyr ACR Rifle
Steyr ACR entry (Matthew Moss)

Check out Vic’s video on the Steyr ACR and an article with further detail here

Steyr-Mannlicher’s bullpup entry uses a gas piston driven rising chamber mechanism which rises and falls to chamber rounds as the action cycles. The Steyr ACR, like AAI’s entry, fires flechettes but Steyr’s are housed inside a cylindrical polymer case. The bullpup Steyr was the second shortest at 30 inches (76cm) and the lightest of the rifles submitted weighing 8.5 lbs (3.86kg).

The Steyr ACR borrows its ergonomics from its conventional forebearer, the AUG. With a moulded green plastic stock and a similar pistol grip, trigger and safety layout. Unlike the AUG, however, the ACR uses an AR-15 style charging handle located at the rear of the sight mounting block. The rifle could be fitted with a variable 1-3.5x optic.

The rifle does not use a conventional bolt, instead it has a rising chamber, as a result the rifle fired from an open bolt. A live round only entered the chamber after the trigger had been pulled, thus reducing the potential for cook-offs. Spent polymer cases were pushed out of the chamber and ejected from an opening just in front of the magazine well.

Steyr's Polymer Cased Flechette round (US Army)
Steyr’s Polymer Cased Flechette round (US Army)

Feeding from a 24-round box magazine, made from the same translucent material used in conventional AUG magazines. The magazine goes from double stack to single stack, to allow it to feed reliably, as a result the capacity had to be shortened. A high capacity drum magazine was planned but not provided for the trials. The polymer case held a fin stabilised 9.85 gr flechette with a moulded four piece sabot which broke up soon after exiting the muzzle. This was identified as a shortcoming as it risked hitting nearby troops.

Specifications (From ACR Program Summary):

Length: 30 inches / 76cm
Weight: 8.5 lbs / 3.86kg
Sights: Iron or variable 1-3.5x optic
Action: gas-operated, rising chamber
Calibre: 5.56mm plastic cased flechette
Feed: 24-round magazine


After several years of development, while the Army organised testing, the four firms delivered their prototypes in 1990. During Phase III the rifles were tested on the specially built Buckner Range – designed to recreate field conditions and create fleeting targets at various ranges. The M16A2 was used as a baseline weapon throughout all of the tests which included safety and engineering testing, lethality tests and testing in field conditions.

Initially planned as an all-services test both the US Navy and Marine Corps pulled out of the ACR program in 1987, leaving the Army and Air Force. With the testing complete it was found that none of the rifles proved to be significantly more lethal or effective than the M16A2 they were pitted against. The program summary put a rather positive spin on the program’s failings, concluding that despite the increased hit probabilities not being attained as a technology base program, the ACR program was a success. The soldiers taking part in the field tests performed better than expected and “the baseline performance of the M16A2 rifle was better than anticipated in terms of hit probability.”

The report concluded that the program had helped push small arms technology forward, noting that while flechettes proved to be too inaccurate for an individual weapon,

“The feasibility of caseless and lightweight plastic-cased ammunition has more than been demonstrated in this program. Few problems were experienced with the [H&K] caseless rifles in the test. The past technical barriers of cook-off and vulnerability have now been overcome. …plastic cases cannot be used in conventional weapons like the M16A2, but with weapons specifically designed for it, complete plastic cases are feasible.”

This last observation is something that is again now being developed in the Lightweight Small Arms Technologies (LSAT) program, which has now been linked with the US Army’s current Next Generation Squad Weapon program.

With the end of the ACR program the school of thought moved towards the use of high explosive and airburst munitions by individual soldiers. The Objective Individual Combat Weapon program ran throughout the early 1990s and while it lead to the adoption of some new weapons, such as the M320 grenade launcher, it also failed in its goal to create an integrated individual weapon system.


Bibliography:

Advanced Combat Rifle, Program Summary, Vol.1, ARDEC, 1992 (source)

‘Revisiting the SPIW Pt.3’, Small Arms Review, R. Blake Stevens, (source)

Our thanks to the collection that holds these wonderful examples of the ACR rifles


Please do not reproduce photographs taken by Matthew Moss without permission or credit. ©The Armourer’s Bench 2018

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