"Environmentally friendly bullets. Non toxic, and kill humans more reliably." Why, oh why, would they waste time, money and effort on a tungsten-based bullet given that "there was no tungsten in the U.S. strategic stockpile, and the bullets, alone, would have accounted for the Western Hemisphere’s entire annual output. The world’s largest tungsten producer—China, which accounts for 88 percent—could not be relied upon in wartime."
We can fairly suppose that the group initially tasked with concocting a "green bullet" either started with a "no limits" attitude (design what's ideal, then accommodate real-world limitations) or just didn't take the task seriously enough (focused on "green", failed to address billions of rounds and battlefield conditions).
At some point, however they started and progressed, they eventually got to some military leaders who imposed real-world limitations: must be affordable, must be practical at ten-orders-of-magnitude scale, must be reliable, and must kill actual enemy combatants better than any prior 5.56 round; "environmentally friendly" isn't enough, and sacrificing our soldiers for it is intolerable.
Result was a "green" bullet that doesn't use tungsten.
