Return of Zombie Ops is a zombie modification for Call of Duty 4, it features content from Black Ops Zombie mode aswell as from Modern Warfare 3 Survival mode and Ghosts Extinction mode. It's set slightly in the future with some weapons from Ghosts, Black Ops series and Modern Warfare series.
Details:
MP5(SD)
9x19 mm Full Auto
Silencer is an option
Looks like G53 a bit.
Well, it is the Black Ops 2 MP5. (The one used in the campaign)