In 1986, the US. Army began the Advanced Combat Rifle (ACR) program to find a replacement for the already aging M16 assault rifle introduced in the 1960s. (In 1986 the Army was on the M16A2.)
The idea was to create a firearm that could double the hit probability of the average soldier and their rifle. This was based on a statistic that the average soldier with an M16A2 could guarantee hitting a target at 45 meters in combat, but out to 220 meters, that dropped to one out of ten shots on target.
This wasn’t the only effort to replace the M16 around that time. The Special Purpose Individual Weapon program ran concurrently, ending in 1990 after costing about $300 million and not resulting in an M16 replacement, but that’s another story.
Of the companies to submit entries, guns from AAI, H&K, Steyr, and Colt were selected for Phase III trials.
Comparatively, Colt’s entry was pretty standard, simply building on what we now call the AR rifle platform and resulted in what was essentially an improved M16.
The Colt ACR was designed to fire both conventional 5.56 NATO ammunition as well as the decidedly unconventional 5.56 mm duplex round.
You can read about the bizarre M198, which was literally two bullets stacked end-to-end in a shell casing in this post from thefirearmblog.com.
Duplex rounds did increase hit probability at close range, since every shot put two bullets down range, but as you might imagine, accuracy at distance suffered, requiring the use of conventional M855 rounds, which made this enhancement kind of useless.
Additionally improvements made for the Colt ACR included a new oil/spring hydraulic buffer to help reduce recoil, which worked quite well. According to this post from armourersbench.com, it reduced recoil by as much as 40 percent.
The rifle also had a reshaped pistol grip and a wildly reshaped hand guard that included a sighting rib, sort of like what you’d find on a sporting shotgun, for snap or instinctive shooting.
The upper receiver included a weaver rail (this was before Picatinny became pretty much standard) so that an early ECLAN 3.5x optic could be attached. A more conventional rear sight / carrying handle could also be attached.
On the muzzle was a proprietary muzzle brake / compensator made by Knight’s Armament, which was designed to reduce the rifle’s report by 13.5 decibels, the story says, and further reduce recoil.
On the back end, the rifle had a familiar looking six-position telescopic butt stock was the same that Colt put on its carbines.
Needless to say ,the ACR was not selected, nor was any rifle from the program, and drifted off into prototype history.
Most of the features developed for the Colt ACR were ditched, though some made their way into the M16A3 and later the M4 carbine, like the selector switch configuration and the now ubiquitous flat-top receiver.
Somehow, that giant ribbed hand guard never quite caught on.
The other entires included the AAI ACR, which was the latest variant of the company’s experimental fléchette rifles, which used a standard 5.56 NATO cartridge firing a 1.6×41.27 mm fléchette in a sabot.
Heckler & Koch put up their G11, which used baseless ammunition with the propellant molded onto the bullet itself, which made ammo smaller and lighter. The K2 version submitted for the test held 45 rounds in a long magazine. The space-rifle-looking gun was never put into production.
Steyr’s rifle also fired fléchette ammunition, but theirs worked differently than AAI’s. Stey’rs ammo used a plastic shell casing instead of molded propellant, which still reduced ammo weight. However, this resulting in a complex firing mechanism that required the entire chamber to move instead of just the bolt.
Neither the Steyr or AAI rifles could be fired in full-auto and were limited to three-round bursts.
The ACR program was also abandoned in April, 1990 after all entries failed to come close to the 100 percent improvement over the M16A2 that the project demanded after a cost of nearly $300 million.
It was followed by the Objective Individual Combat Weapon program, which was discontinued before bringing the promised firearm out of prototype phase. The idea was bold and forward thinking, with the intended rifle being able to engage targets behind cover by using airburst ammunition, which would be smaller than existing grenades and associated launchers, bur large enough to be effective against infantry.