OK Big a few year's ago you would have been right. But the 5.56 is the standard nato round now. There's a number of reasons behind this, but some of the major reason's are the 5.56 is lighter to carry so an infantry section can carry more rounds in the field than was the case with the heavier 7.62. The 7.62 has a longer range ok, but infantry tactics now say that a section should'nt have to engage the enemy at a range greater than 300m. After that range you should be able to call in fire support from either platoon support weapon's ie 50cal, light morter's etc.. arty and air support.
At short distance the 5.56 is more accurate than the 7.62 in that it's not effected by wind conditions as much as the 7.62 because it's a small round moving throught the air.
There's lots more I can give you but I'm off to see Dublin play in a major match in 2 hour's and have to fly.
Regards
Bouncer