Flechette, cartridge length is directly related to the size & therefore mass of the firearm; long action vs. short action. Similar case for the magazines unless these are far, far wider than the brass-case equivalent. The whole point of the endeavor is that they've managed to shrink the fixed ammunition cartridge yet again (by going to more efficient geometry & modern double-base powders to form the 308, then by re-examining realistic infantry needs and optimizing around the much smaller/less powerful 5.56 with the technological advances in pressure curve, powder, geometry, and gas system design), and have also managed to replace yet another metallic component with much lighter plastics.
TCB
. I also think the notion that we can't possibly field another caliber than we do is a bit silly. While it may be annoying to think about the ramifications of more ammo, it isn't like there aren't more types of uniforms or MREs at this point, and I've heard shipping computers are at least twice as fast as they were in 1978. The training aspect seems the largest hurdle to me, if you insist of making every man spend precious time learning on every fielded platform (which if they are otherwise similar, like AR15/AR10 would seem to be unnecessary)
