New Army "Caliber Configuration Study"

[nincomp]

Huh? Steel core bullets in the same form factor as lead core bullets will have a LOWER BC than the heavier lead core bullet. If you maximize the optimum shape for two bullets of the same weight, the denser bullet will always win on BC.

You are correct, assuming that the bullets are of the optimum shape. The problem here is that BC gets better with a better form factor which ends up with longer and longer bullets. When we are looking at magazine-fed weapons, the overall length is limited. This limits the form factor. The perfect example of this is seen when the 6.8 SPC is compared to the 6.5 Grendel. The 6.8 SPC is limited to shorter, lower form factor bullets, so no matter how much the shape is optimized, the BC will be lower than that achievable with the Grendel.

Another good example is the 5.56x45 and the 5.45x39. When they use bullets of roughly the same weight and sectional density (55gr M193 / 53gr 7N6), the 5.45 permits a longer ogive nose, giving a better form factor and BC.

As has been noted by John and Stan, if the performance of a lighter projectile is to match the M80 at long range, the bullet will be too long to fit within the Grendel specs. It is not out of the question that the new cartridge would be the same length as the current 7.62x51 to allow enough powder space and a long-ogive bullet.

In the end, several different parameters need to be balanced, among them are the shape of the bullet and ogive length, bullet weight, case shape and capacity, recoil impulse, etc. Then come the issues of twist rate and bullet stability at low temperatures. When it comes down to it, the goal of Caliber Configuration Study is to figure out what the parameters will be.