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.
Two prototype guns were made. One of them, along with the other three finalist weapons, can be seen on exhibit at the National Infantry Museum in Fort Benning, Georgia, where they were also tested. Prototype #2 is also kept at the Steyr Mannlicher arms factory in Austria.
The Steyr prototype is of a bullpup configuration, in which the internals, along with most of the barrel are placed within a smooth polymer body shell. It is also ambidextrous, due to having a bottomside ejection port ahead of the magazine well, eliminating ejection problems with left- or right-handed shooters. The flash hider on the barrel also features three rows of five compensating holes each drilled on its 1-o'-clock position. It also has an AR-15 style charging handle.
The mechanism is 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 the ejection port. 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 weapon could be configured either for the standard 2x optic sight or traditional iron sights.
Ammunition is fed into the weapon with a 24- or 28-round detachable box magazine that is made of polymer.
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 translucent polymer, reducing the weight by about half of that of the standard 5.56×45mm NATO round. When fired, the flechette rode in the sabot until it exited the barrel.
The ARES-Olin AIWS had also used telescoped ammunition.
The weapon had a tendency to malfunction 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 and disperses, the pieces would be a potential safety hazard to friendly troops or the shooter, bouncing off the ground when firing in prone.
Along with that, due to inconsistencies in the strength of the plastic casings, the flechette tends to leave the cartridge at varying pressures. This, in turn, leads to varying muzzle velocities and changes in trajectory from shot to shot, negatively affecting accuracy. Steyr responded to this problem by experimenting with different materials and combinations of methods.
Another problem lays in the high production costs of the rounds, meaning that very few countries are interested. In addition to this, is that even though spent casings eject from the bottomside of the ACR, the hot plastic can potentially burn the shooter in the wrists, so they would have to wear gloves to protect their wrists.
- 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"
- The Firearm Blog: Steyr Mannlicher factory tour