5.56 (vs. 7.62x39):
-More accurate round (around 600-800 meters)
-Lighter weight
-Relies on high velocity to fragment or tumble round into target
-Higher velocity (62 gr FMJ bullet 3,100 ft/s)

7.62x39 (vs. 5.56)
-Less accurate, less range (around 300 meters)
-Heavier weight
-More bullet mass, able to shoot thru concrete cinderblocks
-Lower velocity (123 gr FMJ bullet 2,400 ft/s)