Based on AAI's SFR/XM19 rifle developed as part of the
SPIW program, the AAI ACR was externally one of the more conventional entrants to the ACR program. A closed bolt, gas operated, full stocked, magazine fed, conventionally laid out rifle. The 1990 ACR Program Summary report describes the operation system as: "...incorporate an 'entrapped gas' operating system. Gun gases enter a cylinder, drive a piston to power the system, and prevent any leakage of propellant gases and residues into the other mechanism parts." Other contemporary sources describe it as innovative, but how it differs from traditional
Long or
Short stroke piston systems already in existence at the time is unclear. ''Jane's Infantry Weapons'' hypothesizes that it replicates the system from the previous Serial Bullet Rifle/Serial Flechette Rifle in which 3 chambers were cycled in and out of alignment with the barrel to give a rapid 3 round burst. They base this on the design being well regarded at the time but being held back by problems with the contemporary flechette cartridge that later development solved for the ACR. Semi-automatic and three-round burst are selected with a two-position selector. The safety catch is in front of the trigger guard and when activated physically blocks insertion of a finger into the trigger-guard. The rifle features a quick change lever for swapping optics, and was provided with a recommended 4× optical sight with a tritium-powered
graticule. Iron sights were also available and the mounting did accept other optics. As with the
Steyr ACR, the
sabots left the barrel at high speed, this was noted as presenting a potential danger to other soldiers, as well as to the shooter if they bounced off the ground when firing prone. Additionally as the cartridge used the existing 5.56×45 mm case, traditional 5.56 rounds could be chambered by the rifle. Due to the light weight of the projectile, the gas port tapped gas earlier in the barrel than conventional weapons. Therefore, if traditional cartridges were used the increased pressure could cause serious malfunction or injury to the user. To prevent this, the magazine was designed to prevent the loading of non-flechette cartridges and the magazine well altered to not fit an M16 magazine. However, rounds could still be chambered by hand. ==Ammunition==