Today’s DTIC document – from June 1973 – covers the early development of the 5.56mm FABRL cartridge, which is one of my favorites. For those unfamiliar with it, the 5.56mm FABRL (Frankford Arsenal and Ballistics Research Laboratory, later rationalized to “Future Ammunition for Burst Rifle Launch”) was a cartridge from the early 1970s intended to reduce the recoil and weight of current and projected rifle designs, allowing for more controllable and higher cyclic rate bursts, as well as lighter ammunition. Key to this principle was the extremely low form factor bullet (lower form factors are better, and the AR-2 bullet had a smoking form factor of .84 i7, compared to M193’s 1.24) which allowed the very light 37 gr .22″ caliber bullet to maintain the same ballistic coefficient as M193, and the same trajectory when fired at the same velocity. Further, the bullet was flat-based, improving barrel life, and in order to reach 3,270 ft/s, only 39,500 PSI chamber pressure was required from the shrotened 1.5″ long case, which opened the door to lightweight aluminum cased ammunition. This resulted in an extremely lightweight round, weighing in aluminum alloy cased form about half that of M193, with the exact same external ballistics. It’s not difficult to see why this was an attractive alternative to other competing future ammunition projects.
The round itself featured a spindly flat-based bullet with an extremely long ogive, a steel penetrator (not unlike the later M855 round) and a plastic core. This projectile configuration would later be echoed in the SS90 round of the early production P90 submachine gun. The 5.56x38mm case was the same as 5.56mm, but shortened to just over 1.5″ long, and was made from both brass and aluminum alloy.
In my opinion the 5.56×38 FABRL represented the concept with the most potential relative to its ambitiousness. Indeed, the Russians would follow the same route during this period, choosing a low form factor bullet for their 5.45×39 round, with a similarly short case. A Limited Analysis of a New Ammunition Concept for Potential Future Rifle Application is a pretty short read, and has something for anyone with even a passing interest in experimental 20th century small arms.