While the 223 isn't the best choice for hunting deer,it is more than capable of doing it. I have killed many of them with a 223,as well as know a lot of others that only hunt with a 223.
Now,I wouldn't even think about shooting a game animal with a "Varmint" bullet,but there are bullets like the 65gr Sierra Gameking,and the 70gr Speer Softpoint that work very well on deer within 200 yards. Both of those bullets will drop them in their tracks with good shot placement.
Does that not depend on the size of the deer and the terrain?? If the angle of your shot is not a perfect broadside or quartering away, don't you think it is not enough bullet? A .243 or .257 Roberts could take Elk, but a similar problem arrises. Having a bullet that will smash bone is pretty good insurance. That usually comes from mass weight and velocity. The heavier the mass weight, the less the velocity needs to be. That does restrict distance since that heavy slow bullet will drop fast after a certain distance, but it still retains energy.
A 70gr .223 at 2900fps mv has 1407fpe at the muzzle. w/ a 150 yard zero it has a 2" drop at 200 yards and carries 1021fpe.
Now, my favorite whitetail carry rifle in 9.3x57 (.366) w/ a 286gr bullet w/ a mv of 2100fps has 2817fpe at the muzzle. 4" drop w/ 150 yards zero at 200 yards w/ 1900fpe. It will hit like a wrecking ball.
My second favorite is 6.5x55. W/ a 129gr (.264) it carries 1560fpe at 200 yards. Both will penetrate better, one from shear weight, the other by SD.
.223 here in the north would just make wounded game.