Please! Don't try head shots, whether with a .223 or a .300 magnum! Deer will react to muzzle flash by jerking their heads.
In the tenth of a second it takes for a bullet to travel the distance you'll end up with a deer whose jaw is shot off 'way too many times.
The only thing more disturbing than finding some young hunter endlessly wandering the woods trying to track a jaw-shot deer is finding the deer itself a couple of weeks later.