You must have the grenade launcher and two types of ammo (e.g., Grenade Rounds, Incendiary Rounds). Put all items in item box. Put the Grenade Launcher in upper-left item slot. Put Incendiary Rounds in upper-right item slot. Close box. Open inventory. Equip Grenade Launcher. Open box. Select Incendiary Rounds from inventory. Press 'A' twice. Move cursor to Incendiary Rounds inside box . Press 'A' twice.

The amount of Incendiary Rounds in the Grenade Launcher will now increase. The number value will be graphically glitched. Close the box and open inventory to fix that, and enjoy the tons of ammo.