The Steyr ACR prototype, designed by Ulrich Zedrosser, makes full use of its bullpup design and telescoped flechette rounds.
This weapon was Steyr's entry for the U.S. Army's Advanced Combat Rifle program in the late 80's/early 90's. The Steyr was one of the top four ACR prototypes that were reviewed in Phase III, but none had managed to meet the army's requirement of 100% improvement over the M16 and therefore none were accepted.
To this day, the Steyr ACR, along with the other three finalist prototypes, can be seen on exhibit at the National Infantry Museum in Fort Benning, Georgia, where the weapons were also tested.
The Steyr prototype employs a unique gas-operated system to cycle through ammunition: instead of driving rounds forward into the chamber and being held in place by a locking bolt, the entire chamber traveled vertically the width of the round. After firing, the expanding gases force the chamber vertically downward, where a new round was placed into it from the rear, ejecting the spent casing out a bottomside ejection port ahead of the magazine. Springs then raise the chamber back into position where it is locked into a fixed block. The firing pin is fixed above the chamber, entering through a small hole and striking the primer to fire. The chamber is normally held in the "down" position, and the trigger releases it to allow the springs to drive it upward and fire.
A very similar mechanism is also seen in the ARES-Olin AIWS, which was an entrant in Phase I of the ACR program.
The ACR's rounds consist of a 9.85 grain carbon steel flechette using a four-part spindle sabot contained in a telescoped configuration with the propellant packed around the projectile. It also featured a side-initiating ring primer. Instead of brass, the bullet casings were made of a lightweight translucent plastic, reducing the weight by about half of that of the standard 5.56x45mm NATO round. Spent casings are ejected from the bottom of the weapon, alleviating ejection problems with left- or right-handed shooters. When fired, the flechette rode in the sabot until it exited the barrel.
The ARES-Olin AIWS had also used telescoped ammunition.
The Steyr ACR could be configured either for the standard 2x optic sight or a traditional iron sight.
The weapon had a tendency to jam and the accuracy and effectiveness of the flechettes left much to be desired, despite their flatter trajectory and lower recoil. In addition, when the sabot leaves the barrel, the pieces would be a potential safety hazard to friendly troops or the shooter, bouncing off the ground when firing in prone.
- Steyr ACR Article
- Wikipedia: Steyr ACR
- Modern Firearms - Steyr ACR - Advanced Combat Rifle (Austria)
- The Firearm Blog article "Steyr’s ACR: The ’80s-Era Teutonic Wonderwaffe You Haven’t Heard Of"