Steyr ACR: Flechette-firing assault rifle

Steyr ACR prototype flechette-firing assault rifle (Photo: Steyr)

Steyr ACR was the Steyr-Mannlicher entrant for the U.S. Advanced Combat Rifle competition. It was considered an excellent example of modern technology, but it could not reach the 100 percent improvement over the M16A2 demanded by the U.S. Army, like the other entries.

The company has said that it represents their view of the next generation of assault rifles, and they will continue to develop it privately. They will probably offer it for consideration sometime early in the next century. At the same competition, there were other rifles, including AAI ACR and AICW.

Advanced Combat Rifle (ACR) Program: From top to bottom, ACR test rifles from AAI, HK, Steyr, and Colt
From top to bottom, ACR test rifles from AAI, HK, Steyr, and Colt (Photo: XY)

Mechanism

The Steyr ACR (Steyr Advanced Combat Rifle) uses a gas system to actuate the breech mechanism, which is quite unusual and built around the special cartridge. This cartridge is of plastic, a plain cylinder with the priming composition arranged in a ring around the inside of the case, just ahead of the base. A fin-stabilized flechette lies inside the case, its fins positioned by the primer ring and the body held by a polycarbonate sabot and surrounded by propellant.

The breech consists of a block that carries the chamber. At the commencement of firing, an operating arm is held back against a spring. This arm is released to run forward on pulling the trigger, take a cartridge from the magazine, and load it into the chamber. The chamber then rises vertically to a position behind the barrel, where a spring catch locks it. Above the chamber is a fixed firing pin, pointing downwards, and as the chamber rises, this firing pin passes through hile in the chamber block and, just as the chamber aligns with the barrel and locks, strikes the ring primer and fires the cartridge.

Steyr ACR (Advanced Combat Rifle) layout schematic
Steyr ACR (Advanced Combat Rifle) layout schematic (Photo: Steyr)

The flechette is driven up the barrel; gas, tapped from the barrel into a surrounding chamber, drives back a piston which is actually a sleeve around the barrel. This drives the operating arm back, unlocking the chamber and lowering it to the loading position. As the trigger is pressed for the next shot, the arm goes forward again, and the cartridge entering the chamber pushes out the spent plastic case of the previous round, ejecting it forward from the weapon. There is no rim on the plastic case, so no obstacle to this forward ejection.

Design

The Steyr ACR is a bullpup design, with the magazine almost at the rear of the stock. The mechanism is enclosed in a plastic outer casing, there being something of a family resemblance between this and their well-known AUG rifle. A carrying handle above the weapon is extended almost to the muzzle, acting as a sighting rib for snap shooting, and iron sights are fitted; a telescope can be quickly attached to the carrying handle. The barrel is rifled with a twist of one turn in 85 inches, giving roll stabilization to the flechette to improve accuracy.

Steyr-Mannlicher ACR (Advanced Combat Rifle)
Steyr ACR (Advanced Combat Rifle) (Photo: Steyr-Mannlicher)

The only real defect to the design is that the flechette tends to leave the cartridge at varying chamber pressures due to inconsistent strength in the plastic cartridge case. Varying pressures mean varying muzzle velocities and changes in trajectory from shot to shot so that accuracy suffers. This, however, is simply a question of testing various materials and assembly methods until a consistent release pressure can be obtained, and, probably, Steyr has already solved this, ready to offer the rifle to the next applicant.

Technical specifications

Manufacturer: Steyr-Mannlicher GmbH, Steyr, Austria
Type:  gas-operated, selective fire
Caliber: 5.56 mm synthetic case flechette
Barrel: 21.25 in (540 mm)
Weight: 7.12 lbs (3.23 kg) without magazine and sight
Magazine capacity: 24 rounds
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