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1. Protect you 2. take small and large game 3. Be low profile enough not to draw adverse attention 4. Efficient enough to allow a decent amount of ammo to be carried along with your other gear?

My SHTF kit is ready with a S&W .44mag Mountain Gun, with .44 shot shells in a speedloader as backup to the 240gr rounds in the cylinder. That's as all inclusive as I have ever come up with, and seems to address your four points as well.
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It's not that life is so short, it's that you're dead for so long.