Tau; It’s a coincidence that I happened to be working on chapter 3 of The History of Modern U.S. Military Small Arms Ammunition, Volume III, by Hackley, Woodin, and Scranton when I saw your post. Here’s a quote from chapter 3, edited slightly to remove references to the figures, that should answer your question:
"…To gain preliminary ballistic data on a new series of low-drag (L/D) 5.56mm bullets being jointly developed by Frankford Arsenal and the Ballistic Research Laboratory (FABRL), the standard 5.56mm case was loaded with a series of long L/D bullets during 1972-1974. The first of these was the “AR2 artillery shape” scaled down to 5.56mm with a long pointed ogive and flat base, but this design was difficult to manufacture, so a compromise shape, called the “Von Karman”, with a less radical ogive and blunter nose was designed. A Nosler machined-steel bullet was also fabricated in the AR2 artillery shape and coated with copper or a dark gray finish.
Examples of some of these loadings include the AR2 bullet loaded to a cartridge overall length of 2.675 in., and a Nosler hollow steel bullet with lead slug and Bakelite base filler, coated with a 0.003-in. thickness of copper and loaded in a case headstamped F A 7 2 with red primer sealant. Others, called the AR2 Short and AR2 Long, were loaded to a cartridge overall length of 2.098 in. and 2.72 in. respectively, in cases headstamped F A 7 2 with red primer sealant. The AR2 Short bullet had a nominal weight of 37 grs., the Long, 43 grs., but weights varied depending upon design and type of slug and base filler used.
Some later AR2 bullets, machined from aluminum and copper-coated, were loaded in cases headstamped L C 7 4 with purple primer seal and sent to the H.P. White Laboratory for testing in 1975. One of the final designs of the AR2 bullet was fabricated in April, 1976, at Frankford Arsenal using a machined, hollow steel bullet, copper-coated, with steel core and plastic base filler, a boattail base to improve accuracy and a knurled bullet seating cannelure. Bullet length was 1.045 in. and weight was 38.8 grs. However, there is no record of this bullet being loaded for ballistic testing. Because these bullets had a lighter weight and used a reduced propellant charge to reach an optimum velocity of 3250 fps, this development eventually led to a shorter-cased series of 5.56mm FABRL rounds (see Chapter 15, Experimental Calibers)." …
Hope that helps. You’ll love the many rare box labels and fantastic drawings by Gene Scranton that are included with every chapter, and this section of chapter 3 is very well illustrated.