Special Purpose Individual Weapon The Special Purpose Individual Weapon ( SPIW ) was a long - running United States Army program to develop , in part , a workable flechette - based " rifle " , though other concepts were also involved . The concepts continued to be tested under the Future Rifle Program and again in the 1980s and 1990s under the Advanced Combat Rifle program , but neither program resulted in a system useful enough to warrant replacing the current M16 . 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 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 . 6×53 mm rounds firing a single dart . The result was a weapon with somewhat less accuracy than the 7 . 62x51mm 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 0 . 22 - inch 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 multiple barrel prototype in order to quickly test the concept . Various multiple barrel rifles entered the project . 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 , fully loaded with 60 rounds . Accordingly , the Army became extremely interested in the weapon . The conclusion of Project SALVO was to adopt the Armalite AR - 15 , which became the M16 rifle . Project NIBLICK . 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 . The final requirements resulted in an over - under weapon , firing flechettes from one barrel , and grenades from the other . It was to weigh under fully loaded with three grenades and 60 flechettes . Four companies responded to the contract tender : AAI , Springfield Armory , Winchester Arms and Harrington & Richardson . AAI continued with the original 5 . 6x53 mm XM110 rounds , while Springfield and Winchester used a new 5 . 6×44 mm XM144 cartridge . The H & R design loaded the dart from the XM144 into their own cartridge design . The H & R design was the most advanced . It mounted the dart between three plastic sabots in a triangular plastic cartridge . When fired , the sabots were discarded early in small " sub - barrels " while the dart continued down the main barrel . Although the rounds were lightweight the weapon itself was not , at loaded , and the weapon was quickly eliminated from the contest . Winchester ' s design used a " soft recoil " stock which absorbed the recoil of an individual round in a spring . The idea was to allow three rounds to be fired before the spring was completely compressed , meaning that there was very little recoil until the end of the burst . This was intended to dramatically increase the accuracy of burst fire , but the system never truly worked , and was later dropped . The grenade launcher portion of the design proved much more interesting , however , and work continued on that portion . The Springfield entry was most interesting for its layout . It used two 30 - round magazines back - to - back in a bullpup layout , taking rounds from the front magazine first . However , their massive magazine - fed grenade launcher pushed the weight to , and it seemed unlikely this could be reduced by much . Oddly , AAI ' s design was the least technically advanced . The flechette gun portion was somewhat heavy , but fired at 2400 rpm . Their grenade launcher was a simple single - shot weapon ; their semi - automatic version was not ready in time for testing . The conclusion of the testing was that none of the weapons were ready for development into a combat system . The AAI flechette portion and the Winchester grenade launcher were both interesting for general development , however . More worrying was the result of general testing of the flechette concept . While the weapons delivered on their promise of extremely high rates of fire and excellent penetration , the rounds themselves were extremely expensive to produce , and the darts could be easily deflected in flight even by heavy rain . Finally , the rounds gave off extremely loud reports and had a huge muzzle flash , making the guns easily visible in low light . A second round of testing followed , with the Springfield model adopting the Winchester grenade launcher with a disposable magazine and a new side - by - side layout for the flechette magazines . AAI ' s design was equipped with their semi - automatic launcher , which was now complete , and a new plastic buttstock / sighting system . Neither updated version proved very reliable and both were over the limit . In 1966 SPIW was put into " maintenance mode " and the M16 adopted . AAI continued development at a low level , and eventually managed to dramatically improve the reliability of their XM19 . However , this revealed another problem : heat buildup in the chamber was great enough to result in " cook off " . 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 . Ironically , 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 Future Rifle Program . In 1969 , the Army started the Future Rifle Program , and invited AAI to enter against General Electric ' s Dual Cycle Rifle , a derivative of the Springfield SALVO design . ( Springfield had closed in 1968 . ) Additional entries with multiple flechettes or bullets ( " micro - caliber " ) were also entered , mirroring the original SALVO tests of the 1950s . However , the program came under attack by the United States Congress as a waste of money , and was forced to scale back . Testing finally began in 1974 . The AAI design never managed to fire more than six bursts before jamming . A possible entrant was the French designed VFIW assault rifle . Close - Assault Weapon System ( CAWS ) . It was revived again during the semi - automatic shotgun - like Close - Assault Weapon System ( CAWS ) project in the early 1980s . A number of designs were entered and offered fairly good reliability , but the entire concept of a large weapon for use out to 100 yards was questioned and the project later dropped . Advanced Combat Rifle ( ACR ) . The concept was revived yet again during the Advanced Combat Rifle ( ACR ) trials in the early 1990s . Several designs were entered , from the simple Colt ACR to the more interesting flechette designs , such as the Steyr ACR . 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 . Modern variants . Since the demise of the SPIW , there has been another attempt to revolutionize the assault rifle by way of a small projectile and a very high rate of fire , namely the West German Heckler & Koch G11 caseless rifle . That project did not result in a production rifle either , as the reunification of Germany put financial strains on the German Army budget , which caused the project to be dropped . A similar project by LSAT is undergoing trials .