I carry about twice as many mags as I usually use, so I've never run out of ammo with my current load. If I were to run out, I'd whip out my speedloader and load two of them back up. Once those were dry, I'd switch to my sidearm and go through its two mags. After that, I'd still have my 3 40mm shells. After that, I'd still have 2 tennis balls.
If I really wanted to turn myself into a walking supply depot, I'd pilfer the mags and pouches from my "guest" loadout and stick those on my back in case some prick on my team runs out of ammo. I've half a mind to do it too, because it would look hilarious and make me weigh twice what I do without any gear.
It's bad enough I still own a hicap; I would never carry a bag of BBs with me on the field.