Masterkey

''This article is a stub for the Call of Duty Series Attachment and Weapon. For more information click here'' The KAC Masterkey is an under-slung attachment that for all assault rifles thus increasing their close range stopping power. First appearing in Call of Duty: Black Ops the successor of the Masterkey, the M26 Modular Accessory Shotgun System appears in Modern Warfare 2. The Masterkey/M26 is unlocked by completing the Assault Rifle Grenadier Challenge, which requires players to get 20 grenade kills with an underslung launcher

Fanon Games the Masterkey/M26 are featured in;

Call of Duty: From the Cold

Call of Duty: Modern Warfare Endgame (Older Assault Rifles use the Masterkey, while more modern weapons use the M26 Modular Shotgun)