The following is from the International Cartridge Collector. No. 289-290 (Jan-Feb 1980) p.4

SQUAD AUTOMATIC WEAPON (SAW) CARTRIDGE DEVELOPMENT AT FRANKFORD ARSENAL (1971-1972)
COMPILED BY WOODIN LABORATORY

During July 1971 a computerized parametric design analysis (PDA) was started at Frankford Arsenal to determine the ammunition concepts to meet the SAW user requirements. Five candidate designs–5.56MM, 6.00MM, 6.35MM, 6.5MM (all using conventional steel or lead core jacketed bullets) and a 6.5MM steel flechette concept were initially studied. The refinement of these efforts during August 1971 resulted in the concentration on two calibers: 5.56MM (.224) and 6.00MM (.243) using either a lead or steel core jacketed bullet. (This was the first use of a math model for small arms cartridge design–the printout of which gave data on basic design parameters such as bullet weight, bullet length, barrel length, muzzle velocity, chamber pressure, basic case dimensions, systems momentum, weapon weight, etc.)

The 6.00MM (.243) cartridge was based on a 80 gr. steel core bullet with a case similar in general configuration to the standard 5.56MM but with a smaller head diameter of 0.372 in. (5.56MM is .378in.) and correspondingly smaller diameter shoulder and a shorter case length of 1.7193in. (5.56MM is 1.756 in.). The design work took place during the September to November 1972 time frame and resulted in a brass case very similar to the original Cal..243 PDA math model computer printout (25 August 1971); however, as with the Cal. .224 design, minor dimensional changes were necessary, particularly in head to shoulder length, to agree with the current fabrication process and to utilize existing case components (5.56MM 2nd draw pieces). The resulting design was recorded on Frankford Arsenal Sketch BCX-I, dated 12 November 1971, entitled 6.00MM Brass Case (Special) (see above). About 800 of these cases, headstamped FA 71, were fabricated in the Case Shop – Building 215 and held awaiting the completion of the design effort for an 80 gr. steel core bullet. There is no evidence to indicate these cases were ever loaded, and approximately 700 were still in storage at Frankford Arsenal in 1977.