Quick: What’s the most advanced infantry rifle that was ever designed, but never got the chance it deserved? I’m thinking about a futuristic weapon from Central Europe that fired advanced, lightweight ammunition and featured a high rate of fire “hyperburst” firing mode. Care to take a guess?

Did you say “Heckler & Koch G11”? Well, that’s one possible answer, but another is the Austrian Steyr ACR, an advanced concept designed to compete against the G11 for the American Advanced Combat Rifle trials of the late 1980s. The Steyr ACR was just as ambitious as the G11: It used a separate rising chamber driven by an annular piston wrapped around the barrel, feeding incredibly lightweight polymer-cased telescoped flechette-firing ammunition from translucent polymer box magazines holding 24 rounds.

The Steyr-Mannlicher ACR rifle in the hands of a US Army soldier during the ACR program. Note small details like the adjustments on the futuristic optical sight, the in-line back up iron sights, and the compensating ports on the flash hider. Image source: The Black Rifle II, by Christopher Bartocci

The Steyr ACR incorporated many small details that made it every bit a match for its ambitious German competitor. Instead of the familiar centerfire primer, the ACR used an annular primer ignited when the rising chamber reached its zenith in the action; this simplified the rifle’s mechanism and also improved in theory the consistency of ignition.

Unlike the G11, the Steyr was production optimized with a simple mechanism and inexpensive polymer housing; estimates at the time put the cost of Steyr ACRs at up to $400 less per unit than the existing M16A2. The ammunition, too, was cheaper; 60% less expensive than existing brass-cased M855, while being only 42% as heavy.

The Steyr ACR’s 5.56x45mm plastic cased flechette round, far left. It fired a 9.85 grain/0.638 gram flechette at 4,600 ft/s (1,400 m/s).

Ultimately, despite its promise, the Steyr ACR proved to be a dead end. The ACR trials it was created for did not result in a procurement program, and all rifles tested remained as prototypes only.

Today, you can still see the Steyr ACR, along with the other ACR entrants, at the National Infantry Museum in Ft. Benning, Georgia.