My thinking has always tended towards: Revolver, 5 rounds in the gun. 2 speed loaders, 30-36 rounds of loose. Rifle, single shot, 40 rounds. Semi-automatic, one 5 round magazine, one ten round magazine, two 20 round magazines. Plus 20 to 40 rounds loose or any combination there of. If it's an urban situation like the aftermath of Hurricane Katrina: Semi-automatic, not less than 4, 20 round, magazines plus as much additional ammunition as I can waddle along under.