Stopping your typical* zombie requires destroying it’s brain. Just how large a caliber round will inflict enough damage to eliminate a zombie’s ability to coordinate it’s movements or track prey? World War Z claims that eventually an incendiary .22 round became the standard choice; but if you don’t have such special ammo, how big a caliber to use?
*WWZ taken as baseline for question; obviously different fictional forms of zombism might have different answers.
I think that you posted this in the wrong place, so I reported your question for a forum change.
If all I can have is a handgun, I’m going for a .50 cal. If I can have a shotgun, I want a 12 gauge Mossberg, with an extended tube.
Honestly, I do think that a gas powered chainsaw would be the best.
I’ve read opposing opinions here…lead pipe tends to lead.
Holy cow…I’m sorry. I’m the idiot, I thought I was in the pit. I’m sorry…please redact my report.
Not if they are infectious. Or heck even if they aren’t; cool as chainsaws are, I don’t think most people really want to be splattered with a spray of shredded zombie meat.
Or if you insist on using a chainsaw on zombies, forget a plain gas powered saw. Go for rockets.
Hornaday Zombie Max ammo starts at 380 auto on the smallest end. Since they are the only ones I know currently making zombie specific ammo I think that qualifies them as the experts in the field.
Der Trihs, you are very right. Chainsaws look very cool, but the ick factor is there. Not to mention that chainsaws are noisy and might attract other zombies to the noise.
I do have cats. Not a lot of brains to attract the zombies, but I’ll bet that if I threw Steve at a zombie, the zombie would be out of action. No eyes left, yanno.
Throwing Steve at a zombie would mean picking him up. I’m not sure if that’s a good idea. For you, I mean.
I’m leaning more and more towards flamethrowers these days, for all sorts of pests. Cockroaches, telemarketers, zombies…if they’ll burn, they might be a target.
Eh, if you are gonna use a handgun, you might want to stick to calibres you can keep control of, especially if you need to hit the head. One of the bigger rounds like .357 would probably do wonders, but I imagine .45 or even 9mm would probably work given how relatively small the head is.
And from what I’ve been told, chainsaws don’t really work on wet stringy things, being designed with wood in mind.
That said, this definitely sounds like something for the Box of Truth 
I’m very confused now, isn’t a .357 smaller than a .45 or am I even more ignorant about bullet size terminology than I thought? (Serious question)
It is true that the .357 has a smaller diameter than the .45, but there’s more to it than that. The .357, though thinner, is longer, and it turns out that both rounds are roughly the same weight, somewhere between 150 to 200 gr depending on the exact type and manufacturer. Also important though is that the .357 is a higher velocity round, typically 1400 to 1600 ft/s compared to the .45 at 1000 to 1200 ft/s. This means that the .357 round has more energy in it.
The round’s “number” does generally refer to its diameter. .357 and .45 are in inches, 9mm is in metric, for example. When we say a .357 round though we don’t just mean any old cartridge that happens to have that diameter. It refers to a specific design with a specific diameter, length, and powder load. Arguably there’s probably better ways of describing it than just picking its diameter, but that’s the way we do it. Sometimes you run into trouble when different rounds have the same diameter, but that’s why we have things like the .45 ACP and the .45 Long Colt which are two different rounds that are not compatible with each other. Some rounds will use an abbreviation like that to differentiate them, others use different methods, like the .30-06 (.30 caliber designed in 1906).
Oh it gets a lot worse than that XD
Aside from .45 ACP and .45 Long Colt, there is also .45 GAP.
And sometimes the numbers are less about accurate measurements, and more about marketing. .38 Special, .357 Magnum, and 9mm Parabellum all have the same diameter, but differ in other important ways (although, fun fact, you can usually fire a .38 Special round from a gun chambered for .357 Magnum, but not vice versa). In this case, the different numbers are more so you can tell them apart easily and not do something silly like buying a box of .357 Magnum to try and squeeze into your .38 revolver (in that case, they won’t fit, just barely, by design)
What can also cause problems is differing powder loads for the same type of ammo. It is theoretically possible to get a bullet that your gun is theoretically chambered for, only for the round to generate too much pressure when it is fired, damaging the gun (sometimes quite spectacularly. And that’s why you wear eye protection on the shooting range.) It’s also possible to have ammo that doesn’t generate enough pressure, which for automatic or semiautomatic weapons, means they might not cycle properly. Usually off-the-shelf ammo will indicate if it is more or less powerful than generally expected, I think. Hand-loaded stuff can be chancier depending on who did the loading.
That said, while chosing what type of gun you want to use to deal with the zombies< I will suggest you get a nice big truck, with a big spacious gas tank, and plenty of room in the back for supplies. Perfect for going over any Zombies wandering about in the street that you would otherwise have to socialize with on foot. 
.357 is (IMO) manageable, recoil-wise, but the muzzle/gap flash is a sight to behold. Especially with the hot Remingon 125gr. JHP rounds.
Longer barrel helps reduce the flash though, right? More time for the powder to burn. I just inherited a .45 Colt officers model. It has a shorter barrel than the standard .45. I’m wondering if that would flash as much as my GP100 Ruger with a 6" barrel.
.357 magnum is my choice for general purpose self defense. Before that tragic boating accident in November '08, I might have owned a revolver and a rifle chambered for that round. I load hollow points, which should be sufficient to splatter zombie brains all over the scenery.
muzzle flash, maybe, but there’s still a nice blast from the cylinder-barrel gap.
When you need a .357 round that doubles as a signal flare, when magnum is just not enough, there’s the .357 Maximum:
http://www.ammo-one.com/357Maximum.jpg
http://img92.imageshack.us/img92/5006/tmp9461gq.jpg
Yeah, sorry, my .357 is enough of a wrist wrecker. I’ve shot .44 magnum a few times, but I don’t go size for size sake.
Back when I used to take people to the range*, I would alternate .38 special** rounds with .357 rounds in the revolver to let people see and feel the difference.
Going back to the OP, it would depend on where you hit, what part of the brain hosts the zombie aspects, and how much damage needs to be done. Unfortunately, zombie movies are terribly inconsistent on the subject. Sometimes the slightest penetration of the brain*** will take them out. But I wouldn’t really go with anything smaller/weaker than a .380 or 9mm****.
It is true what you say about the amount of damage it takes to kill a zombie being inconsistent to say the least.
Assuming we go with your basic any brain injury kills the zombie route I’m going with 22LR.
Several advantages
I dislike the current theme that any quite mild tap to the head seems to kill Zs.
Personaly because of the m velocity I think that even a bog standard 22 RIFLE would take out a Z, though obviously not a short.
Also wouldn’t being enclosed in some sort of body armour make you pretty safe from Zs ?
though they’re supposed to be incredibly strong,
(Another capability that seems to be dimnishing in recent Z stories)
you’d be safe from bites and scratches, and as such safe from infection.
I think that the laziness setting in, in Z stories, where people aren’t worried about being splattered,where Zs are killed very easily in hand to hand, will if left unchecked, result in Zs being perceived as a nonthreat, make it unbelievable that they took over in the first place and ultimately lose the fear and suspense in this genre.
Result nobodys interested in Zs anymore.
If that happens both the story tellers and the audience will lose out.
TV/movie companys, stop the rot now.