Hi,
I have about a third in plastic (MTM) boxes, a third in original or factory style cardboard boxes, and the rest in Ziploc type bags. Bagged ammo allows the most in the big ammo can, but boxed one way or another is easier to organize. Cardboard boxes take more space per given number of cartridges than plastic, but are easier to label on ends as well as tops, so I can figure out what's what if they're stacked vertically in the can.
Rick C