Shotgun Shells are fairly common ammunition items found in every Resident Evil game, though usually less plentiful than Handgun ammunition. They are, as their name would imply, used in conjunction with a Shotgun. The shells are often described as "12 Gauge".
In Resident Evil 6, Helena's Hydra utilizes a larger 10 Gauge Shells rather than the 12 Gauge Shells. The 10 Gauge Shells are exclusive to her as she is the only character that can pick them up.
Resident Evil: Revelations limits the amount for carrying Shotgun Shells and restricts any other Shotgun Shells from being taken if the amount has reached its limit. The player can extend the limit by obtaining an Ammo Bag specifically for the Shotgun Shells. This also applies in Raid Mode, though the Shotgun Shells Bag can only be bought in the store.