22
by wings
Wait, are we shooting the saber-toothed wildebeests first? I'm presuming that they're dangerous too. This also speaks to the fast vs. slow zombies question, as saber-toothed wildebeest have to be faster than even your worst-case-scenario runners. In this case, .357 magnum is probably a better choice than .45 ACP. Flatter trajectory, better range - important in this case.
I'm less worried about the wildebeest indoors, because you can only cram them through the stairs one at a time, and corpses pile up. As soon as one of the rusty scimitars comes available, grab that for backup. In CQB, blades > guns, but even then a scimitar needs a lot of free space to swing for proper effect. Zombie lore suggests that decapitations > deep thrusting wounds, but any CNS hit should count.
Now, if we're allowed to deviate from the OP calibers, I cannot believe that you left out the venerable .45 Colt, CD. Ideal for the wildebeest, not bad for controlled headshots either. Unfortunately, reloading single action under zombie siege isn't ideal. Your original calibers allow moon clips in double action revolvers, so there's that. But high capacity semiauto sure has advantages.
I suspect this might be the best application for the 5.7x28, all things considered. While the noise may attract zombies, a high-velocity, small caliber round might be ideal for headshots, and screw the penetration. Honestly, that might be the best application for it in any defensive situation.