I believe that I mentioned before that I’m editing, including formatting, History of Modern U.S. Military Small Arms Ammunition, Volume III by Frank Hackley, Bill Woodin, and Gene Scranton. I have read the entire book several times, and it is progressing toward completion in the not-too-distant future (don’t ask; I have no idea when it will be done).
Chapter 12 (of 21) deals with 6mm Squad Automatic Weapon (SAW) ammunition. A quote from the chapter’s first couple of paragraphs and a later one follow:
"The ammunition for the 6mm Squad Automatic Weapon system would prove to be the last full scale small caliber development program carried out at Frankford Arsenal before the Arsenal’s closing in 1977. The user requirements for this new weapon included terminal effectiveness parameters at 1000 meters which exceeded the performance capabilities of the standard 5.56mm. Accordingly, a new caliber or a more powerful 5.56mm cartridge was envisioned.
Development work started at Frankford Arsenal on a cartridge for the squad automatic weapon during July, 1971, when a computerized Parametric Design Analysis (PDA) was started 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 jacketed bullets with steel or lead core) 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 in.) and 6.00mm (.243 in.). 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 initial production of steel cases were headstamped F A 7 2 and used a 0.130-in.-thick steel strip with an 875 degree temper. The case was protected by a phenolic varnish finish previously developed at Frankford Arsenal during 1967-1971 for the 5.56mm and 7.62mm steel case programs. During December, 1972, the Pitman-Dunn Laboratory experimented with an extrusion process for forming the 6mm steel cases. To identify these, a modified 5.56mm steel case heading bunter (F A 6 7) was used. About 100 were loaded into ball rounds for tests."
Frank explained to me in an email today that while he was at Frankford Arsenal (as its final Commanding Officer), the late Dave Hughes, author of The History and Development of the M16 Rifle and its Cartridges, visited the Arsenal and located some scrap (F A 6 7) cases in various stages of completion, which is probably the source of these.
Hope this helps answer the question.