Many of the lethality complaints revolve around the bullet and retained velocity. Hornady has recently introduced a 75 grain superformance load for the .223 Remington (
http://www.hornady.com/store/223-Rem...ormance-match/). The muzzle velocity of 2930 ft/sec is 140 ft/sec greater than the standard 75 grain match load.
The 140 ft/sec may not seem much, but it is enough to make that bullet reasonably effective out past 400 meters.