Re: the 5.56 CQB debate: yeah it all depends on the ammo. There are obviously superior cartridges, but 5.56 has the (to me, massive) benefit of ubiquity. No matter what kind of zombie scenario you ever find yourself in, if there's a gun store around or an ammo depot to raid, there will be 5.56 in stock
. You can't say that about a lot of the specialty stuff. .308 (aka 7.62x51 NATO) and 9mm (x19) are similarly really solid choices on universal, unending availability anywhere on the planet.
My 5.56 stuff of choice is Hornady 75gr TAP when I have the choice and the dollars to stock it though. But really, M855 works fine and I stock that too for plinking and emergency supplies. Either way it will get the job done. If you don't think it's an effective deterrent or manstopper, I challenge you to select the worst-performing normal 5.56 round you can find and stand 100 yards away while someone unloads a clip of it into your chest