Having shot a great many fallow deer with a .222. Almost all brain shot. In my fenced paddocks as meat culls. My opinion is it is FAR from an ideal sporting deer cartridge. Deer move their heads constantly. Perhaps in the wild, unaware deer are calmer and move their heads less. A good brain shot needs to be carefully taken with some patience. It is not hard to miss or wound the deer. I had my first jaw shot a week or so ago, but wuickly shot again in the brain. In the wild it might have been well and truly gone. Probably my standard is 19/20 one shot brain kills. I used to use a .22 Magnum as the bang is far quieter. However a good skull shot which misses the brain does not kill. A .222 will create more damage and often shatter the skull or damage the brain. A 6mm would be even better. However where I shoot them, I don't want a larger bullet zinging off anywhere. Same with brain shooting kangaroos. A miss on the brain with a .222 if it hit the jaw, can blow it off. If the roo gets away a terrible death. I think a .243/6mm is an ideal minimum.

A deer can also be killed quite cleanly with a good broadside lung shot. A friend claims his .22 RF placed on a certain dot (fallow deer) will die every time, quietly and cleanly. A .222 round placed in the lungs will probably blow up the lungs and quickly kill the deer. I wonder what the effect is with a 50 gr or 55 gr fragile soft point on a rib? Don't know. I haven't used the lung shot at all, always going for the brain shot.

Supposedly a fair number of "trophy" sambar deer are shot with .222's and .223's in the brain when dazzled by a spotlight, but hardly sporting and not a trophy by any standard.

Just some comments from personal experience.

A reason sporting free range hunting is usually recommended needing something like a .243 and larger. Because of variability of circumstances, lack of time or patience, different shooting angles and aiming points to reach the vitals.