The Shotgun is a weapon attachment in Call of Duty: Modern Warfare 2. It is an under-barrel pump-action shotgun known as the Masterkey, based on the Remington 870MCS developed originally for the M16 assault rifle. It is attached under the barrel of a weapon in a similar manner to grenade launchers.

In Multiplayer it is unlocked for Assault Rifles by getting 20 Kills with the mounted grenade launcher. It is pump-action with a four round magazine, and a capacity of 20 shells. If put onto a weapon with limited close quarters effectiveness like FAL, FAMAS orM16A4, or a weapon that has a small magazine, like the SCAR-H, the custom class becomes more versatile while still allowing for a secondary weapon. This attachment is relatively rare and allows the user to surprise enemies due to its infrequent use. It is also faster to switch to this attachment than it is to switch to a secondary weapon other than a handgun. Unlike most secondary shotguns which fire eight pellets per shot, the Masterkey fires only six pellets. Players should notice that the crosshairs grow larger when the player is moving, unlike other shotguns. This is unusual because none of the other shotguns lose hip accuracy while moving. This shotgun has by far the longest range of all of the shotguns, with its pellets traveling more than double the range of the Model 1887. However, at such ranges it will take multiple shots to kill a target, making it fairly ineffective. The Masterkey benefits greatly from both Stopping Power and Steady Aim.

 The Masterkey is a rare attachment found in the singleplayer campaign. It can be found on the level SOG, mounted to the player's starting M16. In the same level, the Marines will occasionally drop an aM14 with this attachment when killed.