Just do what they do in "The Walking Dead" and slice their heads off and walk away.

I personally believe the best gun to use in a zombie apocalypse is some kind of high-end .22 semi-auto rifle, as the rounds are powerful enough (if you use .22 longs or high-velocity rounds) to go through your typical human skull, the rifle itself is generally lighter than most others, allowing you to carry more food/water/other survival gear, and the ammo is a.) really light, allowing you to carry a good amount of it, and b.) hands down, the most commonly sold round in the USA, found in many lootable hunting stores and hoes across the nation.
Still, a crowbar to the face would do the job nicely.

.22 is only good if you are engaging targets from less than 25 yards and you are a good enough shot to get consistent headshots on moving targets.

Remember that you will have to fight other survivors, and other survivors are smart enough to take cover, and .22 is worthless in terms of cover penetration.

All civilian and military variants of the M14 accept both 7.62x51mm NATO (standard issue in the US military for general-purpose/medium machine guns, notably the M240 and M60) and .308 Winchester (a popular hunting round)

7.62x51mm NATO can stop anything on two legs, even through solid cover. It is common enough for apocalypse use, and the M14 is an excellent rifle in every respect.

Assault rifles and .22s are garbage. Battle rifles are the way to go, and the M14 is the undisputed king of battle rifles.

I concede to your "other survivors" point, and I think we may be talking about different kinds of zombies. I was thinking of the "slow" kind, that can only be taken down by sufficient damage to the brain. I still stand by my original point, however, while I'd prefer to be holding an M14 in an apocalyptic situation, strictly against zombies, I'd prefer a .22, as I'd be more mobile, with more ammunition.
However, speaking of 7.62, I could simply get my Intervention, go FMJ, get sleight of hand, and then 360 quickscope the zombie noobs.

Like^

Too bad the CheyTac M200 is chambered for .408 CheyTac, not 7.62.

While the .22 does have enough power to penetrate the zombie's skull and kill the zombie (I imagine Walking Dead-esque zombies), your accuracy and range (particularly with a blowback semi-auto) .22 is going to be complete shit. Ammunition may be common, but you're gonna need top shelf .22LR for a semi-auto .22 to reliably operate. A homemade silencer is also a possibility, but the complete lack of range, accuracy, in addition to the questionable reliability/durability is enough for me to rule out a .22 as a viable zombie gun.

ITS CALLED AN INTERVENTION YOU NOOB IS SAYZ SO IN CALL OF DUTY!!!11!!!1!!!!!!!1!!one!!1!!!
On a more serious note:
I agree that .22s are not the most accurate at ranges longer than 200m, but I've used an old, low-end varmint gun to shoot pretty accurately at 100m, and a "middle aged" low-end semi-auto to hit pretty accurately at ranges up to 150m. If I wanted to be as mobile as possible, which would probably be in the early stages of an outbreak, I'd still choose the .22. At that point, I'd be far more interested in personal survival than actively going out and killing zombies until I had a nice "base of operations" set up, and a large cache of supplies that were readily available.
Have you read the Zombie Survival Guide, by Max Brooks? Fun, quick, read from which I base my definition of a Zombie.