The AAI ACR was a prototype flechette-firing rifle developed for the Advanced Combat Rifle program.
In Phase III of the program, it was one of the top four prototypes, alongside the Heckler & Koch ACR, Steyr ACR (which also fired flechettes), and Colt ACR. Ultimately, none of them proved to have 100% improved performance over the M16A2.
To this day, it is on exhibit with the other three ACR Phase III prototypes at the National Infantry Museum in Fort Benning, Georgia.
It has a long profile aids the shooter in point-fire situations.
The ACR is gas-operated and used a three-chambered breech system which moved very quickly in and out of engagement with the barrel to allow for very fast cycling (1800 rounds per minute) of a three-round burst. However, there is no provision for fully automatic fire.
The weapon's muzzle carries a rather unusual compensator/muzzle brake system which has been especially designed to work with the flechettes, along with reducing the muzzle blast and noise level. The barrel is rifled with very slow twist - one turn in 85 inches - which gives a fired flechette a degree of roll stabilization and helps accuracy.
As standard, the ACR had traditional iron sights, but the weapon can be configured with a quick-release, 4x ACOG optic scope for longer range engagements.
Like its predecessors, in the form of the AAI SPIW, XM19, XM70, and SBR weapons, the ACR fired fin-stabilized flechettes. The ammunition used the same bullet casing as with the standard 5.56×45mm NATO round, but with a 10.2 grain, 1.6×41.27mm steel flechette encased in a four-part sabot. Compared to standard bullets, the flechettes had a much lower recoil and a flatter trajectory. They are loaded into the weapon via a forward-curved 30-round box magazine.
The flechette idea is not new, as several makers, including AAI, had experimented with flechettes as rifle projectiles in the 1960s. But at the time, the materials and construction of the flechette cartridge were not particularly good. By modern standards, new materials and improved technology has made the flechette concept more acceptable, and the ACR was considered a good design, although not sufficiently accurate for the purpose intended.
The major drawback of the AAI prototype is that it could not use standard rounds, despite its ammunition using the same casing. Because of the mechanism, it is dangerous to fire standard rounds from the gun. As a consequence, they had made the magazine proprietary to the weapon so that the user could avoid loading the wrong kind of ammo into it, though a standard round can be loaded into the weapon manually by hand.
- Colt ACR
- H&K G11
- Steyr ACR
- ARES-Olin AIWS
- McDonnell Douglas AIWS
- Colt M16
- AAI SPIW
- AAI XM19
- AAI XM70