First -
YOU NEED TO CLARIFY YOUR INTENDED USE!
If you want to hunt deer with it and you live in a state that does not allow .223 for deer hunting then.......you'd be stupid to get a .223/5.56.
Here's my recommendations:
1. General plinking, SHTF massive amounts of ammo, competition use and varmint hunting - .223
. Reason: nothing beats cheap .223 ammo and it can be very accurate. Find info anywhere.
2. Hunting medium game, competition use, long range shooting - 6.5mm Grendel
. Reason: Widest variety of medium game bullets loadable in AR magazine length without severly degrading velocity, 500 ft-lbs at 900 meters, all production rifles can shoot 1/2 MOA or better with factory ammo. Find info at Wikipedia - the online encyclopedia
http://en.wikipedia.org/wiki/6.5_Grendel
3. Hunting big game, cracking engine blocks - .50 Beowulf
. Reason: Most popular and widest availability of the big bores for AR15, proven capability and reliability (note: some collapsable stocks can't stand the recoil). Find info at Wikipedia - the online encyclopedia
http://en.wikipedia.org/wiki/.50_Beowulf
4. Self-defense or military anti-personnel use alone - 6.8 SPC or 6.5 Grendel
. Reason: a) the 6.8 SPC was designed to penetrate 1" and then massively fragment using Remington 115gr OTM and Hornady 115gr OTM ammo - great for anti-personnel but not so great for hunting - you can get 110gr and 115gr SP and TSX ammo that is good for hunting but there are no other bullet weight selections in factory loads for hunting, the heavier bullets impinge too much on powder capacity - cheapest ammo is $0.75 per round so not SHTF unless you have $$$$$$. b) 6.5 Grendel will massively fragment after 2" penetration when using Black Hills 123gr ammo - $1.00 per round - same SHTF comment as above - you can go with Wolf Gold 6.5 Grendel ammo at $0.50 per round in the self-defense role but expect more hunting round style performance.