TLDR ; Skeeter was Right back then.. If the parameters are still sort of close, Still my choice today .
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Let's get crazy with our parameters for a minute . Alone in the wilderness , with only what you carry on your back is one thing. End of the world zombie apocalypse is another . Access to retail ammo supply chain is a factor . And one gun/ lower w/ multiple bbl/ uppers may or may not he within the spirit of " one gun , .
So lets say variously : Single shot .22lr rifle w/ brick of ammo , AR-15 configuration of choice, 12ga pump , AR-15 with 5.56 and ( 6.5/ 6.8/ 7.62x39 ) upper . 6.5 best ballistics, 7.62x39 widest availability .
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But Skeeter was right with 4in .357 Mag . Splits the difference to be sorta plausible for both CCW , and large-ish hunting , while having readily available ammo at all increments from mouse fart to full house .