The Springfield Armory SPIW is a prototype assault rifle that was designed for the Special Purpose Individual Weapon program.
The Springfield prototype had different designs from 1963 to 1966, with the 1964 bullpup variation being the most notable for its 60-round magazine that is loaded behind the pistol grip. However, despite passing the length restrictions, the weapon proved too hefty, as it was four pounds heavier than what the weight restrictions had required.
Hard as they tried, the problems seen in the SPIW could not be solved. As a result, the M16 continued to see use while the SPIW project was left abandoned.
As of the time of writing, the 1964 prototype can be seen on exhibit at the U.S. Army Ordnance Museum.
The original version of the SPIW from 1962 version was of the bullpup configuration, but did not have a grenade launcher.
The 1963 version was made in both a "conventional" (magazine in front of the pistol grip) and a bullpup configuration. The conventional-style SPIW can be equipped either with a single-shot or three-shot clip-fed grenade launcher. The bullpup-style SPIW can also be equipped with either a single- or three-shot grenade launcher.
The 1964 version, the most well-recognized model, was made in a bullpup configuration, due to the magazine being behind the trigger. The 1964 SPIW used a distinctive 60-round staggered box magazine, which is essentially two 30-round mags in one shell. This unique magazine has two sets of followers and feed/retention lips. It fed from the frontal magazine first, followed up by the rearward mag. The weapon had a catch that held the rounds in the rear stack down slightly as long as the front stack was full. When the 30th round is fired and the front column was expanded, the catch holding the rear column was disengaged, allowing the rounds in the rear column to be caught by the bolt and then pushed through the front feed lips with a divider keeping the round engaged with the feed lips and up into the chamber. It also features a 40mm grenade launcher fed with a 3-round box magazine, along with a bipod and a carrying handle.
The 1965 version of the SPIW was made in a conventional-style configuration and had a wooden stock. It also fires the XM216 round, and used a blow-forward, single-shot grenade launcher.
The 1966 version, which is of a similar configuration, was noted to have a 50-round detachable box magazine.
The SPIW used the XM144, a 5.6×44mm caliber flechette round designed by Irwin R. Barr. The 1965 model and later used the XM216, also a 5.6×44mm caliber flechette round. The grenade launcher component fired 40mm grenades.