I've shot a lot of deer (approaching 400), and quite a few with the .223. Even more with a .22 Hornet. And a number were taken for necropsy/studies.
The short and long of it is:
1. The .223 has a narrower wound cavity than larger diameter bullets. (The volume of a circle increases dramatically with an increase in diameter).
2. Penetration is apt to be seriously compromised if larger bones are hit.
Combine the two above and it can result in a lost deer.
I occasionally take head shots, but it is usually on undisturbed feeding deer, or one that is alerted and looking at me. I have even killed some (depredations permit) with a .22mag RF, but shot placement is hypercritical.
I killed two deer this past Sat. (opening day in GA) with a .223. (Rem mod7, w/Hornady 55gr SP over 25.5gr of RL-15. The larger, a 100lb doe took the bullet broadside through the ribs at 35yds with a ~1/2" exit wound. I over shot the heart, and hit just behind it. The deer ran as if spanked for ~40yds, then stopped, and then fell over. Took about a minute to expire thereafter. Second was a yearling spike buck I mistook for a doe. It took the bullet broadside at ~100yds broadside into the humerous of the left shoulder. The bullet fragmented on the bone and shards of bone and bullet fragments entered the chest cavity lacerating the heart. Deer bolted at the shot and ran approx. 50yds before expiring.
What was wrong with the performance? Absolutely NO blood trail, except where the deer fell. A larger deer than we have here could have possibly soaked up more than one of the .223's, and still been lost.
Thankfully, I returned on Monday with my .35Rem and killed a 125lb doe, and on Tuesday, I used a .30/30 and took a doe and a 180lb 10pt buck. With equal shot placement on the latter two, the doe probably would not have died any differently, except for a generous blood trail from the 150gr RN Corlokt. The 10pt would have been a different story......................
The buck most assuredly would have been lost because I broke both humerous bones in both shoulders, and ventilated the rib cage and nicked the heart. The .223 probably (at least the 55gr Hornady SPt.) would have failed to adequately penetrate and taken the far leg out from under him which kept him from going any further than the 90yds he made on two hind legs.
And, consider that the .30/30 itself is not considered to be a real power-house of a deer cartridge.
If you persist in using the .223, consider using either the Hornady 60gr Soft Pt.(my favorite!), or, the 63gr Sierra Semi-Pt., or the Winchester 64gr Soft Pt. In my experience, these will give you the best possible performance with the .223.
I defer to use the 60gr Noslers because they cost as much for 50 as the Winchesters in component form cost for 250!
You won't always be as lucky as you have been. The first deer took with a .223 took it 5 Remington 50gr PHP's from a Mini-14, and took a round from a .357mag revolver 2 hours later to put him away after I got permission to go onto adjacent property to get him. Meat was not edible!!! I refused to use a .22cf on deer for over 10yrs. until I started working with a co-worker on a depredation permit at a local airport who was alternatively using a .222 and a .243. Even at that, my .257 Roberts was substantially "muy grande" gun than the .243 (My .257 75gr HP @ 3,550fps vs his .243 80gr bullet at 3,200fps. The Roberts has a bit more "slap" at 350yds! The 100gr .257 never failed us! I consider it a .30/30 with the trajectory of a .270. Just about right for 100-200lb whitetails, and the single Mule deer I've taken.
Use enough gun !!!!