The Special Purpose Individual Weapon (SPIW) was a long-running United States Army program to develop, in part, a workable flechette-firing "rifle", though other concepts were also involved. The concepts continued to be tested under the Future Rifle Program and again in the 1980s under the Advanced Combat Rifle program, but neither program resulted in a system useful enough to warrant replacing the M16. Project SALVO The idea of a flechette-firing 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 way to provide high firing rates. SALVO had a small number of "duplex load" weapons developed, where two bullets were stacked, while Springfield Armory and Olin/Winchester both entered multiple barrel firearms. Even before the SALVO tests, Irwin Barr of AAI Corporation 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 —excellent given their light weight—but the dispersion of the darts was so great as to make them only marginally 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 .308Win (7.62×51mm NATO) rounds, but with equal penetration and a trajectory so flat it could be fired with no sight adjustment out to . 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. Project SALVO began in 1951 and was based on the assumption that firing multiple projectiles would increase the probability of hitting the target. During World War II, an infantryman with a rifle at an average engagement distance of under combat stress expended 10,000 rounds for one hit. Consideration to lighter ammunition types and rifle/grenade launcher combinations was