One of the early brass cases discussed below. Dummy loaded with inert salt (sodium carbonate) to simulate the correct weight
Dummy loaded with inert salt (sodium carbonate) to simulate the correct weight Parkerised case
Ordinary lacquered steel case Tracer
Plain steel cases, first specimen with no headstamp en second one with a very faint F A 7 3 headstamp. Frankford Arsenal experimented with different steel case finishes during November 1973.
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.
This was a Frankford Arsenal project started in July 1971 in order to determine ammunition concepts in order to meet the requirements of the SAW (SQUAD AUTOMATIC WEAPON) program, using a computerised parametric design analysis (PDA). It was the first time that a mathematical model was used for small arms cartridge design. Five original designs were studied, but in August 1971 the study concentrated on two calibers, namely 5.56mm and 6mm with lead or steel core jacketed bullets.
Early ballistic tests were done on the 222 Magnum case necked up to 6mm, but during in April 1972 it was found that they could not consistently achieve the desired velocities with commercial bullets. Therefore a larger capacity case was needed The standard 5.56mm case has a diameter of .378in. and this was favoured by Frankford Arsenal, because they could use standard equipment during the initial case forming process. Various designs were done on paper with different case lengths. As can be seen from the above specimens, the bullets used had longer bearing surfaces and overall length, which also influenced the case design. As a result of unsatisfactory results with the 222 Magnum case, Frankford Arsenal did studies on case designs with a head diameter increased to .410in. which at that stage was unusual, because the dimensions were larger than the 5.56mm but smaller than the 7.62 NATO, meaning that existing machinery would have to be modified. On the 11th of May 1972 representatives of the R&D divisions of the munitions and weapons agencies, together with the Small Arms Systems Agency (SASA) decided that the specifications of the new cartridge would be:
a. Cal. .243 (6.00mm).
b. Case Head Diameter .410 in.
c. Case material – steel with a brass backup.
d. Projectile – 105 gr., L/D of 5, lead core.
e. Case taper 0.1746 inch per inch (similar to 5.56mm).
Different materials were used in the case construction. At that stage the US Department of Defence had a “copper saving policy” and therefore the SAW program complied with that directive. However, 1,500 brass cases were delivered in July 1972, although steel and aluminium cases were already under development.