The ARES-Olin AIWS (short for "Advanced Individual Weapon System") was a prototype bullpup assault rifle that was entered in Phase I of the Advanced Combat Rifle program.
Because of technical issues, the weapon was withdrawn from the competition.
The AIWS used a gas-operated rising chamber mechanism, similar to the Phase III Steyr ACR prototype.
When the weapon is charged and ready to fire, the chamber is sealed in a lower position by the trigger sear. When the trigger is pulled, the chamber moves upwards, releasing the firing pin slide. The firing pin locks up the action as it struck the primer, firing the weapon. As the projectile passes the gas port, the expanding gases push the firing pin and rammer slides backwards while the chamber drops into the feed position. With the crossfeed completed, the slides are fully rearward and ready to begin the ramming procedure. The slides then move forward, initiating the ramming and ejection process.
The AIWS uses a 5mm caliber (5×44mm in Phase II, 5×54mm in Phase II), 45 grain saboted tracer round in a telescoped configuration that feeds into the weapon using a 65-round plastic belt carried by a drum. Initially, the cartridge consisted of a brass casing with a plastic end cap at the front, and the projectile sheathed in a three-part sabot. The later version of the cartridge used a plastic casing with a brass battery cup, again with the projectile sheathed in a sabot and a plastic end cap.
The telescoped ammunition concept using plastic casings was also seen in the Steyr ACR.