Through the history of modern small-arms development the concept of ultra-high velocity ammunition has risen to the top of designers in both East and West. An example of the ultra-high velocity ammunition is the flechette.
A metal dart, the flechette is often sabotted in single rounds to be fired out of individual rifles. More beastly variants are container rounds- hundreds of flechettes packed into tank rounds. But it is in the assault rifle that the flechette has seen periodic development.
One such weapon was the AO-27- a 1961 designed Cold War flechette rifle. A select-fire gas operated assault rifle, the AO-27 was a fairly conventional rifle with removable 30 round magazine. What went into the magazine was something else.
The sabotted round as a dart in a typical brass case. However the dart was wrapped in a two piece sabot which allowed it travel down the barrel of the rifle as a normal round would. When the sabotted flechette reached the muzzle, the sabot would peel away from the flechette allowing the dart to continue forward to its target.
Contemporary American efforts into flechette developments produced similar results in the SPIW which fired steel darts at velocities over 4,600 feet per second. For reference modern small-arms ammunition reach velocities of 3,500 fps. These high velocities and simple hardened construction was also an attribute desirable to soldiers who sought a round that could penetrate barricades or dense foliage, and then continue on towards the target. Another bonus for the infantry carrying the flechette ammunition was it had a reduced weight.
An important footnote to the development of flechette ammunition was its legality on the battlefield. Questions as to whether the flechette would maim have been raised by human rights organizations.
While flechette development has occurred on and off, it has never reached the battlefield in rifle form.