if you stick with the AR platform the 6.8 SPC works and has been called the 270 short. there is a 450 Bushmaster and Remington has their AR-30 round which performs maybe between 30-30 and 308. If you go to the AR10 platform you can get the 243, 308, and 338 Federal. Although it can be used, I do not think the 223 is appropriate. There is even some question about its use by the military, which is why the 6.8 SPC was developed. If you have one I can see the desire to use it and within a certain range and with good shot placement it will kill deer. But if you're walking into a gun store and ask for deer rifle I doubt you'll be shown an AR-15.