Of all the four designs, the AAI design was the most basic. The AAI design was also the only design to continue using the XM110 round provided. The flechette portion was somewhat heavy, but the weapon fired at a rate of 2400 rounds per minute in burst mode. It also had a single shot grenade launcher mounted underneath. Eventually, the AAI's rifle was considered for development.
After a few months, the AAI design, now with a complete three-round grenade launcher, was tested. However, it was not reliable and the M16 rifle continued to be used, and the project was allowed to die.
While none of the designs were successful, the grenade launcher mounted on the AAI prototype was considered successful, giving birth to the famed M203 grenade launcher.
The AAI design was the least advanced of all four designs. It was considered to be the lightest, simplest, and the most durable.
The 1964 first-generation prototype of the SPIW used a three-shot, lever-action grenade launcher on the underside of the barrel. The grenade launcher component could hold two shots in a tube magazine, and the third in the chamber.
The second-generation prototype developed in 1966 made some improvements on the previous generation. The first is a new stock made of plastic. The second, is that it uses the 5.6×57mm XM645, a redesign of the previous XM110 round. Because of the extra 4mm in length for the round's casing, the drum magazine and action stroke were made slightly larger. In addition, as another means of reducing the weapon's weight, AAI developed the DBCATA disposable grenade launcher system. Launcher-plates for DBCATA came in both single-shot and three-shot capacities. The three-shot DBCATA launcher-plate used a harmonica-style loading system.
1967 to 1968 saw AAI introduce a family of third-generation SPIW weapons. This included a belt-fed light machine gun, a point-fire rifle with a centrally mounted bipod, a short-barreled variation, a bullpup submachine gun model with the magazine behind the pistol grip, and a redesign of the 1966 prototype with a single-shot DBCATA launcher-plate. Due to complaints about the muzzle flash and noise level of the weapon, a silencer was added to the barrel.
The last variation of the SPIW had the capability to have its foregrip changed out for a grenade launcher component that costs less than DBCATA. The grenade launcher is fed from single-tube or three-tube magazines with standard 40mm grenades loaded in them. The magazine feeds into the launcher from the side, with the magazine having a gear track that hooks onto a rail in the launcher.
The first-generation AAI SPIW fired the XM110, a 5.6×53mm caliber flechette round designed by Irwin R. Barr. Later generations of the weapon used the improved 5.6×57mm XM645 round.