In response to an "improvement over M855 lethality from 0 to 500 meters" you wrote.
if that was the case then someone definitely didn't try to think inside the box very well. M193 ball would increase lethality over M855 penetrator rounds quite easily.
M193 ball has better lethality than M855 only when there is enough remaining velocity to cause the "fragmentation" effect during terminal performance. That velocity drops off quite rapidly, and so from an M4 length barrel you are looking at 200 meters or less.
I suggest you spend a little more time actually studying terminal ballistics at ranges before you come back with a flippant and demonstrably wrong answer. If you had said Mk262mod1 ammunition then you would have a much better argument to make.