I love these 223 threads.
I have a 223 wssm that is frighteningly accurate. (three rounds same hole @100yds) Before you jump on me, I'm quite aware that this is a "magnum" round. But regardless, I can't wait for deer season to open up so I can shoot one with it. Why? Because I can. Didn't Gatehouse write something to that effect?
IMHO, the definitive shot placement is the neck. There is more than enough energy to quickly and effectively break the neck and kill a deer. In fact, if you do a search you will find an article detailing the effectiveness of the .223 (not wssm) on deer. Most, if not all, were neck shots. If you get a chance, look it up. It's a great read.
And yes, I agree with the others that stated, bullet type, shot placement and distance are the big three to consider - regardless of the caliber you choose.
Now get out there with your .223 and let the air of a big one! Don't forget to post up some photos for the rest of us!
Cheers!
p.s. yeah I know i posted this one before...but I'm just so damn proud of it