Testing now:
Pre-additemevent: .880 Magnum Rounds= 6/18
After-additemevent: .880 Magnum Rounds= 6/18
...HOWEVER. The player gains a pack of the ammo to thier inventory, so it can be ctrl-W tossed out to where the player can grab it, though as soon as it's grabbed it'll act like regular ammo/items picked up (ammo will disappear and go into ammo inventory instead of the item slot, however.)
Not quite the effect I was hoping for, but it's a intresting note nontheless.