This thread is funny.
It is my belief that a 9mm PPC is in fact the perfect zombie gun, not a fall back.
Consider this: a gun capable of getting good head shots out to 50m, easily suppressable, easy availability of a common ammunition, low recoil, long term durability or easy availability of parts depending on the type of PPC you have, OR you have a smaller spare parts kit to lug around to cover the high wear parts compared to many other guns.
For instance, a beretta M9 SBR'd with a carbine stock conversion hits almost all of the above criteria. SBR glocks with a stock conversion works great as well. Some factory PPCs already have a great track record for reliability amd require few spare parts to keep them running for a very long time such as uzi's.
Suppression is a must to be able to clear a local area without calling more zeds in from the surrounding area. A 5.56 is just too damn loud even suppressed and the .300blk is simply added weight and ammo availability from battlefield drops is lame as hell compared to a PPC.
Given the scenario described, a well manufactured, SBR'd PPC with suppressor, reliable red dot, and tac light, would be my go-to.