OK, mates, this is one of those "let your imagination run wild" scenarios.
It's the end of the world as you know it, and you're forced into a survival mode. You're not in some exotic locale: you're in your own neighborhood. (Not saying that your neighborhood's not an exotic locale, but you know what I mean!).
You can have one air rifle - or air pistol, for that matter - as your only gun. It can be any make, model, caliber, vintage, power level, cost, etc. but it MUST be an air gun.
So, what's your choice?
Jim