The Special Purpose Individual Weapon (SPIW) was a long-running United States Army program to develop, in part, a flechette-firing "rifle", though other concepts were also involved.

SALVO had a small number of "duplex load" weapons developed, where two bullets were stacked, while Springfield Armory[1][2][3] and Olin/Winchester both entered multiple barrel firearms.

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.

In SALVO testing they were found to be able to penetrate one side of a standard steel helmet at 500 yards (460 m)—excellent given their light weight—but the dispersion of the darts was so great as to make them only marginally useful.

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 400 yards (370 m).

Project SALVO began in 1951 and was based on the assumption that firing multiple projectiles would increase the probability of hitting the target.

[4] Since the Army was by this time only interested in fully automatic weapons, Barr suggested that they build a multiple barrel prototype in order to quickly test the concept.

[5] Meanwhile, the Army's Operations Research Office, ORO, had been working on Project NIBLICK, follow-on to SALVO to develop a modern grenade launcher.

Interested in the original shotgun-type shells, they recommended development of a dedicated flechette-firing weapon combined with a grenade launcher, the SPIW.

Finally, the rounds gave off extremely loud reports and had a huge muzzle flash, making the guns easily visible in low light.

Changes in the Army command structure and the adoption of the M16 made interest in the SPIW fade, and eventually the project was allowed to die.

AAI's original "simple" grenade launcher turned out to be a major success: it was selected as the M203 in 1968 and became a common weapon under the M16.

The concept of firing flechette ammunition was revived for the last time during the 1986 Advanced Combat Rifle (ACR) trials in the late 1980s.

Although the basic problem of a single-dart flechette round had finally been solved by this time, none of the weapons entered offered the 100% improvement over the M16 that the Army tests required.