shots to the body i completely agree with you....I would beg to differ greatly with regards to head shots.
little piece of semi unrelated trivia: the 5.56 as the round was actually never intended to kill, but to wound thus taking 5 people off the battlefield instead of just the one.
I'm sure you could find a greater percentage of people surviving a .22 to the brain than a 5.56, but you could find examples of both. Also with regards to survivability of a head shot, you're looking at it from a human perspective, with all of our powers of healing both naturally and assisted, as well as the will to live which is undeniably a huge factor in surviving a catastrophic brain injury. The zombie will have none of these factors influencing their survival.
truly your only argument is skull penetration, which admittedly is a fair one, but one that is rendered moot by the speed and accuracy that a double tap can be accomplished with a .22.
example: you are within 20 feet of a zombie, you engage with a head shot that fails to penetrate because of angle, unnatural bone density of the skull...take your pick. The force of that round, even if it did not penetrate into grey matter, is going to crack that skull all to hell, giving the follow up shot much less resistance to pass into the brain thus destroying the threat. Also the force of the blow may knock the zombie off its feet, allowing a quick advance and pointblank shot to be accomplished.