User blog comment:Willbachbakal/Singleplayer Weapons: Revolver Shotgun/@comment-3412492-20110220090515/@comment-1330314-20110220162412

Sorry about that. Clip is a generic (and, in this case, not even correct) term to describe a weapon's ammo magazine (i.e. how much ammo a weapon can hold, as opposed to how much ammo you can carry on yourself). So basically, the Drum Magazine upgrade allows you to fire eight shots from the Shotgun without having to reload, while the Ejectable Drum upgrade allows you to reload your entire clip near-instantly.