Ada Wong uses this shotgun in her mission to retrieve a Dominant Species Plaga sample. It lacks a buttstock and instead has a pistol grip, a shorter barrel, and a vertical foregrip on the pump. It has a firepower of 5.0, can fire every 1.53 seconds, takes 3.03 seconds to reload, and has a capacity of 12 shells. Ada's Shotgun cannot be upgraded and since it does not have a stock, it is fired from the hip instead of from the shoulder.
It is possible for Leon to get Ada's Shotgun in his scenario. For some reason, Ada's Shotgun is located out of bounds in the cave at the lake where Del Lago was fought. Using the Ditman Glitch and climbing the ladder of the Merchant's building then jumping down, Leon can stand in mid-air. This can be used to go out of bounds. If Leon runs against a wall, he can lower his altitude. You cannot find it in the Gamecube version for obvious reasons.
When Leon uses Ada's Shotgun, it will look like the first Shotgun he acquires and behave the same. However, it won't appear in the merchant's sell list and the damage is the same as Ada's Shotgun.