The heavier bullets have a better BC, so though they start out slower, they maintain their velocity better over long distances.  Being heavier, they will have less deflection from the wind.  Also, you're comparing a match bullet (made with much higher consistancy standards) to FMJ bullets, so obviously accuracy will be much better in most cases.

Terminal ballistics (i.e., combat effectiveness) would be fairly poor at 300m regardless of what load you use; 5.56mm has a limited effective range, especially with shorter barrels.  Not something you have to worry about when punching paper.

-Troy