Project SALVO
The idea of a flechette-based individual weapon started in earnest during the Army's Project SALVO. SALVO had earlier concluded that a small weapon with a high rate of fire would be considerably deadlier than the large "full power" weapons being developed in the 1950s, and followed several lines of investigation to find the best solution. SALVO had a small number of "duplex load" weapons developed, where two bullets were stacked, while Springfield and Winchester both entered multi-barrel weapons.
Even before the SALVO tests, Barr had been developing single and multiple flechette cartridges. The Navy became sufficiently interested in the concept to provide him with some development funding from the Office of Naval Research, resulting in a 12 gauge shotgun shell firing 32 flechettes. The Army later added funding as well, and AAI was invited to SALVO. In SALVO testing they were found to be able to penetrate one side of a standard steel helmet at 500 yards --excellent given their light weight-- but the dispersion of the darts was so great as to make them not very useful.
Further development continued by adapting a Winchester Model 70 rifle with new XM110 5.6x53mm rounds firing a single dart. The result was a weapon with somewhat less accuracy than the 7.62mm NATO rounds, but with equal penetration and a trajectory so flat it could be fired with no sights out to 400 yards. Better yet the rounds were very light, and had almost no recoil in comparison to even the .22 caliber weapons under development. This meant they could be fired at extremely high rates of fire, from a very lightweight weapon.
Since the Army was by this time only interested in fully-automatic weapons, Barr suggested that they build a multi-barrelled prototype in order to quickly test the concept. The resulting "burst simulators" were tested in 1961, and the general conclusion was that the light weight of the flechette meant that it could be fired at extremely high rates of fire, the baseline being 2300 rpm, from a weapon of only 3.5 pounds, fully loaded with 60 rounds. Needless to say, the Army became extremely interested in the weapon.
