The AAI Serial Bullet Rifle (SBR for short) was a prototype, U.S.-made assault rifle.
AAI Corporation developed the XM70 rifle to participate in the Future Rifle System project in the 1970s. However, the weapon has proven to be too expensive to manufacture. With that, AAI took the design of the XM70, and improved upon it, leading to the SBR. AAI then entered it into the Advanced Combat Rifle program, where it would later be redesigned into the ACR.
The SBR was constructed from glass filled, injection molded nylon components, which make the rifle lighter and more durable than other models.
The SBR was designed to fire two types of ammunition. One is the standard 5.56x45mm NATO round. The second is a 4.32×45mm caliber saboted flechette round. The idea is that the difference of loads would give the weapon the increased lethality in close quarters encounters. However, the 5.56mm NATO would prove better for longer range engagements.