A guns effectivness has many variables. Rate and volume of fire made the M1 Garand superior to its contemporaries. The M16 / AR-15 is a very reliable, field serviceable rifle that has a high rate and volume of fire and is more accurate than many of its contemporaries.
The 5.56 NATO might not have the puch of a .30 cal round but you can carry many many more rounds. So in a fire fight the gunners shooting larger calibers run out of ammo sooner.
5.56 in not perfect but if it sucked so bad do you think the US military would keep using it?