I haven't used a .223 in a while but I know it can take down deer. My first three deer (spikes at 50-100 yards) were all killed with a Savage .223 using Remington 55 grain bullets. Each one dropped at the spot. My brother has killed 4 deer with his .223 (2 doe, an 8 pointer, and a 5 pointer) all at 100 yards.
All it takes is proper shot placement, the right load, and a good shooter.