If S.H.T.F. the 1st thing I'd want is either a rain-suit or umbrella. 
If limited to only One Air Rifle [going with the actual title / question]
I would have to say, I'd go with either a multi-shot PCP w/ built in pump:
[like the FX Indy or the Nova Freedom - and in .22 cal] Or a springer.
Never running out of air would be a huge plus, Built-in pump PCPs make the most sense,
so those options would be just as nice if not better than going with a springer, and,
they'd offer more than just a single shot.. a Huge factor if we're talking zombies too.
Caliber wise - I figure .22 because they're pretty efficient as far as size/weight,
their ability to cut through side-winds better than .20 and .17 cal, and,
they're still easily found / readily available at any sporting goods store or WallyWorld..
Just make sure that if you go to WallyWorld for your ammo,
to do it before they convert it/them into FEMA camps.
Sam -

If limited to only One Air Rifle [going with the actual title / question]
I would have to say, I'd go with either a multi-shot PCP w/ built in pump:
[like the FX Indy or the Nova Freedom - and in .22 cal] Or a springer.
Never running out of air would be a huge plus, Built-in pump PCPs make the most sense,
so those options would be just as nice if not better than going with a springer, and,
they'd offer more than just a single shot.. a Huge factor if we're talking zombies too.

Caliber wise - I figure .22 because they're pretty efficient as far as size/weight,
their ability to cut through side-winds better than .20 and .17 cal, and,
they're still easily found / readily available at any sporting goods store or WallyWorld..
Just make sure that if you go to WallyWorld for your ammo,
to do it before they convert it/them into FEMA camps.

Sam -
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