Argue until you're red in face but I believe the lowly .22 caliber long rifle rimfire cartridge is the ultimate survival caliber. Teamed with a Ruger 10/22 it can't be beat. This video proves the round is fatal out to 440 yards.
If you faced a long walk and had the option, which would you prefer to hump - five hundred rounds of .22 or five hundred rounds of any other caliber...think about it.