This is a step by step guide to getting this glitch to work. Follow these instructions or it will fail.
Requirements: You must have the grenade launcher and two types of ammo (e.g., Grenade Rounds, Incendiary Rounds)
1) Put all items in item box
2) Put Grenade Launcher in upper-left item slot
3) Put Incendiary Rounds in upper-right item slot
4) Close box
5) Open inventory
6) Equip Grenade Launcher
7) Open box
8) Select Incendiary Rounds from inventory
9) Press ''A'' twice
10) Move cursor to Incendiary Rounds inside box
11) 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. I assume the value exceeds the possible amount, so the game automatically reduces the rounds to 240 to prevent data corruption. Enjoy.
This also works for other types of grenade ammo... But why you'd want anything other than Incendiary Rounds is beyond me.
NOTE: This code does not work in the PAL version of the game.