Link to article on ACR entries.
The McDonnell Douglas AIWS (Advanced Infantry Weapon System) was one of the ACR prototypes that did not make to Phase III of the program. The AIWS used lockless "chiclet" rounds. A saboted flechette was positioned in the center of the cartridge with the powder compartments on either side, with the "lockless" chamber closed off at the breech end with a slot at the top and bottom of the barrel. Ammunition is fed into the chamber from the magazine located on the left side of the weapon and a sliding pressure sleeve closes off the barrel’s openings.
The ARES-Olin AIWS also didn't make it past Phase II of the program. The AIWS uses a 5mm caliber (5×44mm in Phase I, 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. 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.
