Steyr Advanced Combat Rifle

The Steyr-Mannlicher entrant for the U.S. Advanced Combat Rifle competition was considered to be an excellent example of modern technology, but, like the other entries it could not reach the 100 percent improvement over the M16A2 demanded by the U.S. Army. The company have said that it represents their view of the next generation of assault rifles and they will continue to develop it privately and will probably offer it for consideration some time early in the next century.

The rifle uses a gas system to actuate the breech mechanism, which is quite unusual and which is 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 which carries the chamber. At the commencement of firing an operating arm is held back against a spring. On pulling the trigger this arm is released to run forward, take a cartridge from the magazine and load it into the chamber. The chamber then rises vertically to a position behind the barrel, where it is locked by a spring catch. Above the chamber is a fixed firing pin, pointing downwards, and as the chamber rises so this firing pin passes through a hole in the chamber block and, just as the chamber aligns with the barrel and locks, strikes the ring primer and fires the cartridge. The flechette is driven up the barrel; gas, tapped from the barrel into a surrounding chamber, drives 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 of the weapon. There is no rim on the plastic case, so no obstacle to this forward ejection.

The rifle 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, so 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.

The only real defect of the design, as revealed in the U.S. tests, 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 it is probable that Steyr has already solved this, ready to offer the rifle to the next applicant.

Steyr ACR Specifications
Operation Gas operated
Caliber 5.56mm
Projectile Synthetic case flechette
Barrel length 21.25 in. (540mm)
Unloaded weight 7.12 lbs. (3.23kg)
Modes of fire Safe, semiauto and full auto
Magazine capacity 24 rounds

This information was authored by Ian Hogg and first appeared in his book entitled Modern Small Arms, ISBN 0-7858-0018-2