All the ammo in the world won't do you any good if you can't take it with you in an emergency. I found a way for me to grab one handle and off I go. If I have more time, I'll grab individual cans with dedicated calibers, but if I need to go as light as possible, as soon as possible, I go with this:
(1) .50 cal can with (3) multi-compartment trays. Load it up with your different caliber handgun, shotgun and long gun ammo and it's ready to roll. It won't hold enough to defend against the entire zombie horde, but it will get you started.
* I'd like to share this little tip with the non-Co-Op crowd as well, so will also be posting it in the regular survival section. If this violates any rules, Mods, please feel free to delete it there.