You must simply have a maxed out a certain type of ammo in your inventory. e.g.: 7.62 NATO (max. 100). Then you just pick up the same type of ammo from crates, closet etc. Switch out your maxed out ammo into the crates etc. The ammo will be maxed out. To add more, pick up and separate your ammo in your inventory and repeat. Works for all types of ammo including Rockets.