Masterkey

Masterkey is a weapon attachment only found in Call of Duty: Black Ops and Call of Duty: Infinite Warfare. it is only found on the Pack-a-Punched version of the AUG for black ops and on a variant of the Erad in Infinite Warfare.

Use
Masterkey is a small shotgun that is attached to the player's gun, in this case the masterkey is quite powerful compared to some shotguns in zombies mode. It can also be useful in saving ammo, as when you use the masterkey it does not effect ammo from the gun it is attached to making it act as a 3rd weapon (4th if you using Mule kick).