Some info from this site:
http://www.ak-47.us/AK-47vsM-16.php
The 223 ammunition it uses. The rifle has enough power to kill humans or deer if not bears and cape buffalo, is accurate out to several hundred yards and, with heavy (75 - 95 grain) bullets and quick-twist barrels, can shoot to 800 - 1000 yards. It is accurate and allows rapid, well-aimed fire.
That might answer your questions on whether or not its good to shoot lions with. Bears have thicker skulls and are more dangerous!
They should technically call it 5.56mm since .223 is a lower powered round for varmit hunting with sporting rifles, not M-16's/AR-15's which use a more powerful round.