I like this idea, I was about the make a post similar to this, with a few extra ideas. You should be able to find loose bullets as well as ammo boxes. For example, you can find a small amount loose bullets, say 1-8, that can stack up 25, then top of this you can a ammo box (ammo specific) that can hold 50 bullets, found with anywhere between 0 -50 bullets in it and it takes up one spot in your gear. The loose bullets can be loaded into the box to save space and/or then loaded into empty magazines. This idea could just be adding an extra level of nothingness but I feel it will give a greater range of items to scavenge for and add to the whole what to keep and what to drop dilemma. As for where to find loose bullets/ammo boxes/magazines I would suggest that you have a small chance of finding them around the destroyed military vehicles that are in most towns, as I could imagine soldiers dropping ammo/mags as they are frantically loading guns while being overrun. On the point of magazines you could have gun specific ones (Glock takes a Glock magazine with 9mm ammo) or calibre specific magazines of different capacity (9mm guns take Glock, M9, PDW and MP5 magazines with 9mm ammo) ? This last part is me just spit balling/rambling at the end of the post.