Picking up the shell gives 4 ammo. Picking up the box of shells gives 10 ammo. Picking up the weapon gives 6 ammo. The max capacity is 100.
Can be found on any level, with the exception of Saving Juanita, Saving Claude, Saving Esmerelda, and Saving Winnie and Mai. It also respawns indefinitely in the room when battling the 31st Century Fox and the Viggo Industries Harrier MK III.