Phil B,
There are a lot of variables to consider to answer you question.
If you are looking for a one shot drop kill with any firearm your are going to have destroy the central nervous system everytime you shot, every time! That is the only way you will get the one shot drop kills.
Is the .223 capable of doing just that given the parameters you laid out.
The question is can you place your shot in the exact right spot each and everytime.
I have shot and dropped coyotes in their tracks with everything from a .22 lr to a 404 Express to a .40 S&W. I have also shot coyotes with those same calibers and had them run off like they were missed.
Maybe I shouldn't quite count that one shot with the .40 S&W. I shot it at about 40 yards as it was walking along beside my truck out in a wheatfield. Dropped like it was pole axed. Walked out in the field pickend it up and carried it back to the truck to take home and skin. tossed it in the back and headed home. As I pulled in the driveway, I looked up in the mirror and the coyote was looking back. Got out the 580 Remington, opened the canopy and stepped back. When the coyote fell out of the back of the truck, I put a .22lr in it's ear and it never moved again. you just have to hit that central nervous system everytime!
When I skinned that coyote I found the 40 cal bullet had passed just under the spine but above the lung an stunned it. Elmer Keith talked about this phenomenon in big game numerous times in his writings.
