Ah, for the love of Mary! Zombies are ridiculous!
I've never understood the zombie fixation, nor have I partaken in a Z thread, but here goes...
Has it been established as to whether or not they are fast or slow? I seem to have missed it...
I'd say a good mag-fed .22mag bolt gun ( like the CZ or the Ruger, both have iron sights, in case of scope failure) with a good compact 1.5-5x scope. Use the lower power for 3m out to 50-60m, the higher powers out to 175m.
The benefits are:
No recoil, good, surgical, skull popping capability out to 175m, one could carry 1,000 rds compactly and with ease, low auditory and visual signature, short bolt throw & no loss of sight pic due to non-existent recoil make for reasonably quick follow-up shots.
Detractors:
There aren't any to speak of really...except that the lack of range/power of the little.22mag, means that you are going to have to pick some really good shooting positions that are secure, yet have good E&E access/egress, just in case.
Oh yeah I'd pack a good heavy pistol or revolver for the up close actions( The Oops! I messed up and got suprised type), one that will bust major bones and cause good structural damage, because no matter how impervious they are to shock/ exsanguination, I'm assuming they will fall to broken hips, femurs, spines etc and be less of a threat and be easy tgts due to lack of mobility, for when one can come back and finish them off........ what a crazy thread!
PS: I know the rifle choice is crazy, but I would think of it as large predatory varmit hunting, so why not use a fun caliber??