AAI SBR (4.32mm-Flechette).

AAI had been designing prototype weapons utilizing flechette loads since the 1950's, with their Special Purpose Individual Weapon (SPIW) which paved the way for future projects. The Future Rifle System (FRS) was the company’s effort to produce a replacement for the M-16 rifle. Initiated in 1969 the FRS Project had four different prototype weapons developed, the Serial Bullet Rifle grew out of one such weapon-the XM70. Facing a considerable expense in the production of the XM70 by the mid-1970's it was decided to try a different approach. The SBR was an attempt to improve upon the XM70 design for the governments Advanced Combat Rifle project. The SBR was made from injection molded and glass filled nylon components to increase durability and reduce overall weight. AAI's SBR was designed to fire two types of ammunition to increase it's desirability during the tests. The first round was a 5.56mm bullet and a 4.32mm saboted flechette round. It was believed the difference of loads gave the weapon the increased lethality in close quarters encounters and yet the bullet would be better for longer range engagements. The design however was not retained through testing and soon was phased out for a caseless design which would comprise the companies next candidate for the ACR prize.

Picture from Giovanni Tendas